No, surprisingly London didn't go simulcast when they switched over... They are still using the 4 multicast sites that the EDACS system used.
You say you have full bars... perhaps he signal is too hot at times? Maybe try using the ATT option, or try it just using the rubber duck for a bit and see if it's any different. With issues like this it pretty much comes down to trial and error to see what works best in your particular case.