i think £2250 is probably a pretty good deal right now, id say it was worth £2500 in the middle of summer. The 2.2i does goes for a fair bit less than the 3.0i i find, which would probably be worth £3000 - £3500 for the same age and mileage.
to be honest these are a lot of car for how much they are - early z4s are very good value considering the performance on offer, but they are certainly getting on a bit now, and they are still in good supply on the secondhand market, rust is just starting to affect the earliest cars now, but the engines are strong, should easily make it to 200k if you keep it serviced.