Not much better. I believe a bunch (all?) of ESA launches go from French Guiana, so my guess is that might be the easiest place to go for orbital launches for a British company.
Ascension would be more likely, given that it has a runway intended as an emergency runway for the Shuttle, and it has a bunch of communications facilities (GCHQ, NASA, ESA, BBC Atlantic relay) as well as US and UK military facilities.
ESA is not in the same situation and e.g. Norway is a member of ESA and Canada sits on the ESA council under a Cooperation Agreement.
Lets not be silly, as much as brexit was foolish, the UK will still work in the ESA closely and will still be on very good terms with the vast majority of nations on the planet.