Having owned 2x R56 minis in the past (a Factory JCW, and a JCW Cooper S), they are great but age has now caught up with them and they do really need to be bought with a good warranty, and you will be compromising massively on space (they are tiny!). A fJCW really is the pic of the bunch if you can find a good one, but I'm not sure I could recommend one over a Golf if you already have one.
Getting a good price for your car will be difficult because nobody knows what the market currently looks like. Plus you'll potentially be rolling into a car that could end up being a worse proposition for you due to having instant negative equity from the dealer mark-up and the depreciation curve.
It's understandable to start looking for 'cost cutting' when things look bad, but unless you have too, stick with what you have is MO. In 2011 I sold my automotive love (a 1M!) by panic selling when I thought I would be made redundant, but in the end I wasn't and to this day I massively regret it, not just financially!).
For the sake of £6k, it would be too risky for me. Factor in negative equity, cost to change etc etc in another car and realistically you are probably only going to be saving £3-4k, and that's if you get what you want for yours.
