Being London based, I do this often. If going into LCY, then my preference is for flying LX and connecting in ZRH. If you’re going to LHR, then AC/UA can be easier as there’s also the arrivals lounge at LHR T2 for a shower and most UA hubs have multiple daily services to London. You’ll get Points, but not QPs.

As to the original question, yes two hours should be fine. Theoretically you’ll have no security or immigration as it’s NS to NS but FRA is just a mess.