My sister is on the Fjords cruise on 29 May and lives near Dover. She is advising people NOT to book the high speed train as it has a bus replacement service which is a pain in the backside at best!
Just go from London Victoria, it really doesn't take that much longer .. We're not talking Japanese bullet trains for the High Speed ok, it's just a little faster on some of the route, not even all of it!
Taxi from Folkestone to the Dover port is going to be around £20.
For the return journey to Heathrow, look at National Express, a bus service that is relatively cheap. Or, just get on the train again, although I wouldn't fancy traipsing around London with my luggage, it's bad enough as a commuter with my laptop bag!!