15000 UKP (22000 USD?) tends to eliminate most of the cars on your list unless you're willing to get one used. You might be able to scare up a used Honda S2000 that fits your budget. Likewise, you can probably get a 3+ year old BMW Z3, although based on my experience you'd be asking for a world of unreliability if you did so.

I drove a friend's Boxster once. I'm only 5'11" tall yet with the seat all the way back I still didn't fit very well in the car. I had to use the wheel telescoping thing to make room for my knees. I've had no trouble fitting into Miatas or MR2s.

If you seriously want fun-but-impractical, then check out a T-Rex three-wheeler. Mad, crazy thing.