Cashing out is about as easy as cashing in, with similar rip-off fees. You can do SEPA both ways (talking from experience with Coinbase and Kraken). Not that you're asking, but I'd recommend Kraken over Coinbase (from the minimal small transactions I've done with both, YMMV). Just remember there are a lot of steps to go from fiat to bitcoin (especially GBP, an additional GBP->EUR, I don't think there's an exchange that accepts GBP), with a percentage fee or flat fee every step of the way (using market prices typically not in your favour). It's unfortunate but that's the way it is.

Whatever exchange you pick, you'll need to give them some personal details, and if it's more than a few thousand, probably scans of your passport. I wouldn't be deterred by that, it's the ones that don't require that for large transfers I'd be wary of.

It's unlikely that brexit will mean the UK will stop using SEPA, there are many countries not part of the EU that are part of SEPA now. Either way, that decision is years off.