Been thru London a few times but never with firearms. Primary reason is the direct flight we have there arrives in Gatwick and we have to change airports over to Heathrow to get to Namibia. Can't check firearms all the way. Have to claim them and hire a licensed transfer agent to take them to the other airport. Last time I checked, this was about $600 each way for two rifle cases and we had to travel separately and not with the courier service.
We could find alternate routes from the US to Heathrow to avoid the transfer courier but, by the time you add the extra stop/layover to get there and back and the BA firearm carriage fee we could find better alternatives. One guy I know goes to Namibia every couple of years and just rents/uses a loaner. Goes via London and does the transfer. Spends the day going from Gatwick over to Heathrow but has to hustle to make the connection home. Missed his flight one time because "the tubes" went on strike. Cost him a lot of extra coin to rebook because he missed his flight.
If you can book so you don't have to change airports in London, it might work for you. London used to be a fun stopover....has changed a lot in the last few years......and not for the better.
