If you want to go Paris cheaply and are in no hurry then using the coach/bus services may be your preferred solution.
Flixbus, OUIBUS and Eurolines are three major bus lines traveling between London and Paris. Info below is taken from www.european-traveler.com
1. Flixbus Coaches:
Flixbus, Europe’s largest intercity bus operator, has taken over all Megabus service on continental Europe and is usually the cheapest for bus travel between Paris and London. Ticket prices generally range from £20 to £50 but deals of around €15 are often easy to find.
2. Ouibus Coaches:
Ouibus has up to eight coaches per day traveling in each direction between Paris and London. The lowest one-way fares on Ouibus are around €30, which is slightly higher than on Flixbus.
3. Eurolines Buses / National Express:
National Express / Eurolines Buses, which has a much larger network of intercity buses in the UK, have around seven coach services daily between Paris and London. One-way fares from London to Paris on National Express / Eurolines buses are from €20 to €40. Discounts are available for return journeys booked online – returns for around £50 are fairly easy to find.
If you are not time sensitive, then Eurostar Snap is an option. You pick a day and either morning or afternoon and then get told the train time the day before. (It lets them fill empty seats.)
There are seats at £25 within the next couple of weeks, and that leaves you in central London/Paris.
Credit:stackoverflow.com‘