It depends on the amounts.

For sheer convenience I'd go for withdrawing cash from a Bitcoin debit card like Xapo or Bitpay. There may be a small loss on the rate but at least there's no risk of your bank getting arsey. I've got both and they work perfectly.

Bitpay fixes the BTC rate when you deposit it. Xapo fixes it when you actually spend or withdraw it.

Other than that many people open a Fidor UK account and withdraw to it from Kraken. Forget the GBP market on Kraken, only use the EUR one and convert with Fidor.

For larger amounts I'd forget exchanges and look into an OTC trade rather than an exchange and either alert your bank beforehand or open a separate account just in case.

It's bleeding pathetic that the UK is still such a desert for this.

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.

Why can't you just have the funds transferred to you via Faster Payments using a site like localbitcoins to place your ads? Every time I want to sell some BTC I find the UK traders and sell to them. I get my GBP in minutes. It's really simple.

OTC = Over The Counter. A broker pairs you with a buyer and the sale happens between both parties. You don't have to dick around with exchanges flaking out. This is how the really deep pockets take positions in BTC without moving the price. The exchanges are too small.

I'm not totally sure who the biggest OTC broker is in the UK. I would probably start here - https://www.coinfloor.co.uk/over-the-counter and ignore the institutional bit, they've offered this to regular folks too.

The biggest US operations will be - https://genesistrading.com/   and - https://cumberlandmining.com/  and I assume they can do things with UK banking.

I've also come across a couple of Brits using this  https://gemini.com/areas-of-operation/ and wiring money back to the UK but that must be expensive and slow. They also do a daily auction which is semi OTC ish in nature.

You can always sell to a local buyer exchanging btc for cash. Just meet in a cafe next to a bank, so you don't get robbed.

Cheapest quickest method I have found is Kraken BTC to Eur then SEPA to bank account who transfer to GBP automatically.

And it's very easy, simple and also hassle free once you've set up your bank account. Nothing to worry about cashing out. 

Using a site such as localbitcoins.com will be the most effective method for withdrawal within the UK. Providing you use the escrow service alongside choosing a reputable exchanger.

Jolly good, a rare positive post.