Snugs will be at CanJam and can take a digital scan of your ears on-site. It takes a a week or two for them to turn these into custom eartips.
If what you need is a physical impression that can be sent to an earphone/eartip maker of your choice, then I think Noble have in the past had an audiologist next to the booth who can probably help with that, if she is there again this year.
Alternatively you can book with
Amplifon in Wigmore Street to get impressions taken. The location is walkable from either Oxford Street or Bond Street stations, either of which is easily reachable from Waterloo.
For some ear impressions, the material (or scan) goes deep enough that it may be necessary to have yours ears cleaned first if you have not had this done in a while. I've used
Clear Ear Clinic for this, and while costly (£80 if I remember), they do extremely thorough work. They are just a block or so from Amplifon, above.