Tricky one! I bought my Series 3 with the idea of keeping it for many years. It's, now, only done 58k miles; full history, new cam belt etc...(Graphite grey, black interior). BUT, I may be tempted by a Ser 4 now. So, yes, if £10k appeared, I think I'd sell.
I love owning this car, even though it's an occasional drive, so it won't be long before I'd return! Will be abroad for a few months soon, which is persuading me a little. Start saving anyone?