I checked it and, unfortunately, you can’t send your comics to London, but only drop them off. Meaning that if you don’t live in London, your only option is to send everything to Florida... The cost of return are pretty high and it’s almost not worth it to do it unless you send a lot of comics at the sane time.
The shipment cost for my last submission (I’m living in Switzerland), was 150$ for 8 comics, which is f****** expensive. Indeed, CGC only give you the FEDEX choice (in Switzerland at least) which will cost you a minimum of 95$ (If you submit enough comics, otherwise the price will be between 120-180$). To that, you need to add the cost to send the comics to CGC (which was 55$ for me)...
Believe me, do not send less than 7-8 books for each of your submission, when living in EU, or you will regret it.