The fastest route is by a Deutsche Bahn ICE train from Frankfurt to Brussels and then Eurostar from Brussels to London. If you intend to do this, book with Deutsche Bahn as soon as the tickets go on sale (usually 92 days before travel) as the cheap through tickets sell out fast. If you miss out on the cheap ones from Deutsche Bahn, look for separate prices for Frankfurt-Brussels on DB and Brussels-London by Eurostar, and also through Frankfurt-London fares on Eurostar. Either may be cheaper than the full fare from DB. (Apart from a higher fare, a disadvantage of buying a through ticket from Eurostar is that they can't reserve a seat for you on the ICE from Frankfurt to Brussels, although the trains rarely fill completely.)
Be aware that Eurostar services have a minimum 30-minute check-in time due to airport-style security. If booking separate tickets for the two legs, I would leave at least a 60 minute gap.
Also, make sure you get off at the right station in Brussels. The ICEs call at two large stations in Brussels, Noord/Nord and Zuid/Midi. Zuid/Midi is the second one and the one you take the Eurostar from.