Here's a trick: If you know of something worth doing in Earl's Court, it's a bizone (1 and 2) station. Underground tix to zone 2 (3 & 4) are only GBP 2.10 compared to something like GBP 3.50 for zone 1. The r/t savings is enough to get you a pint of beer.

The FCC lines can be horrible. The transfer can take minutes but it could also take the better part of an hour or more as noted- with possibly lots of walking, especially if the transfer involves a trip to/from the Victor Zone of T4.

At any rate, it's not U.K. customs that takes time, it's immigration.