Upvote:5
Documentation requirements are detailed by GOV.UK where the only mention of email is to specify that travel booking confirmation (can be email or copy of tickets). Otherwise the requirement is All documents must be originals and not photocopies and there are several references to letter.
emails are never anywhere normally accepted as original since so easy to replicate without any indication of whether original or copy, and so easy to forge.
However, IF your activity is covered by a Standard Visitor visa's conditions then you don't need an invitation.