UK Transit Visa

You do not need a visa for a "layover" at London Heathrow. [A "stopover" in an international context lasts for more than twenty four hours.] You may certainly make an airside transit, and as I remember the timing of flights from SFO to SOF at LHR will give you time to do this.

If you entered the USA within the last six months on the visa in question, you may also make a landside transit (this is required for a change of airports or an overnight stay).

Information as of  19MAY15 / 0826 UTC
National Pakistan (PK)          /Embarkation USA (US)
Transit United Kingdom (GB)     /Destination Bulgaria (BG)
ALSO CHECK DESTINATION INFORMATION BELOW


United Kingdom (GB)    

TWOV (Transit Without Visa):
Visa required, except for Passengers holding confirmed onward
tickets making an airside transit at London Gatwick (LGW),
London Heathrow (LHR) or Manchester (MAN) on the same calendar
day to a third country (excluding Ireland (Rep.)).The
following conditions must be complied with:
- passenger must arrive and depart by air; and
- passenger only intends to transit through the United
Kingdom; and
- passenger must hold all documents required for the next
destination; and
- passenger must not leave the transit area; and
- passenger travels with a document listed in the following
warning(s):
- ***Warning*** Passengers may make an airside transit if
holding a valid entry visa issued by Australia, Canada, New
Zealand or USA. (SEE NOTE 57398)
NOTE 57398: E-visas or e-residence permits are only
accepted for airside transit when the airline is able to
verify it with the issuing country.
- ***Warning*** Passengers may make a airside transit if
holding a valid "D" visa issued by an EEA Member State
or Switzerland. (SEE NOTE 57398)
NOTE 57398: E-visas or e-residence permits are only
accepted for airside transit when the airline is able to
verify it with the issuing country.
- ***Warning*** Passengers may make an airside transit if
holding a valid biometric visa issued by Ireland (Rep.)
endorsed "BC" or "BC BIVS" in order to transit to a
destination other than Ireland (Rep.). (SEE NOTE 57398)
NOTE 57398: E-visas or e-residence permits are only
accepted for airside transit when the airline is able to
verify it with the issuing country.

Visa required, except for Passengers holding confirmed onward
tickets passing through United Kingdom immigration to make a
landside transit to a third country on a flight that departs
before 23:59 the next day. The following conditions must be
complied with:
- passenger must arrive and depart by air; and
- passenger must have no purpose in entering the United
Kingdom other than to pass through in transit; and
- passenger must hold all documents required for the next
destination; and
- passenger must pass through United Kingdom Immigration; and
- passenger travels with a document listed in the following
warning(s):
- ***Warning*** Passengers may make a landside transit if
holding a valid visa which allows entry into Australia,
Canada, New Zealand or USA and traveling as part of a
journey to, from or transit through the country that has
issued the visa.
Landside transit is not available if holding a US Visa Foil
type "YY" or "ZZ" endorsed "Not a visa. Foil prepared at
DHS request" nor the US Adit stamp worded "Processed for
I-551" (temporary form I-551).
- Not applicable to nationals of Syria.
- ***Warning*** Passengers may make a landside transit if
traveling from Australia, Canada, New Zealand or USA
provided transiting the United Kingdom less than 6 months
after the date they last entered Australia, Canada, New
Zealand or USA with a valid visa for the respective
country, even though the visa may have expired at the time
of transit through the United Kingdom. (SEE NOTE 57399)
Landside transit is not available if holding a US Visa Foil
type "YY" or "ZZ" endorsed "Not a visa. Foil prepared at
DHS request" nor the US Adit stamp worded "Processed for
I-551" (temporary form I-551).
NOTE 57399: E-visas or e-residence permits are not
accepted for landside transit.
- ***Warning*** Passengers may make a landside transit if
holding a valid common format "D" visa issued by an EEA
or Switzerland. (SEE NOTE 57399)
NOTE 57399: E-visas or e-residence permits are not
accepted for landside transit.
- ***Warning*** Passengers may make a landside transit if
holding a valid biometric visa issued by Ireland (Rep.)
endorsed "BC" or "BC BIVS" and traveling to Ireland (Rep.).
(SEE NOTE 57399)
NOTE 57399: E-visas or e-residence permits are not
accepted for landside transit.
Additional Information:
- There is no passport control on traffic between Great
Britain and Northern Ireland and Ireland (Rep.).
United Kingdom (GB)

Vaccinations not required.


Bulgaria (BG)

Passport required.
- Passports and/or passport replacing documents must be valid
for at least 3 months beyond the period of intended stay.
Passport Exemptions:
- Holders of emergency or temporary passports.

Visa required, except for A max. stay of 90 days for holders
of a double or multiple entry  C  visa issued by a Schengen
valid for the period of intended
stay. (SEE NOTE 56110)
NOTE 56110: The max. stay is granted within 180 days.
Visa required, except for A max. stay of 90 days for holders
of a double or multiple entry short visit visa issued by
Croatia, Cyprus or Romania, valid for the period of intended
stay. (SEE NOTE 56110)
NOTE 56110: The max. stay is granted within 180 days.

Additional Information:
- National visas issued by Bulgaria are only accepted when
contained in a valid travel document.
- Where Schengen visas exempt the holders from a Bulgarian
national visa, valid visas issued by a Schengen Member State
in full, invalidated travel documents are
accepted if accompanied by a new travel document.
- Visitors are required to hold proof of sufficient funds to
cover their stay and documents required for their next
destination.
Warning:
- Passports and passport replacing documents issued more than
10 years prior to date of travel are not accepted.
- Visitors not holding return/onward tickets could be refused
entry.


Bulgaria (BG)

Vaccinations not required.