Welcome to the forum!
£12K should get you a nice car, there is lots of info on here about different specs so have a search through. If you want a UK car they only came in GT spec or not GT spec so there isn't too many options to worry about. Jap imports have many more options but can be equally as good. GT spec basically gets you leather seats, cruise control and an uprated (although still not great) Stereo.
personally I would get a nice Azure blue 54 reg GT with a low mileage... but I would say that....
To answer your questions...
whats the average mpg id be gettin on motorway driving and around towns? - you'll average mid 20s at a guess, less than 20 is possible in stop start town traffic, over 30 at constant motorway speed
any big services i should look out for? - Servicing isn't too bad, there are 3 levels of service P1, P2, P3. P3 being the most expensive. Depending who you go to the prices range from £150 to £400, which is a steal for a modern car with this sort of performance! There is no cambelt to worry about as its chain driven.
difference within BHP with difference models? - 3 different BHP levels, its been covered several times in the last couple of weeks, have a look...
i very much like to drive long journeys, but will occasionaly enjoy a good blast too, is the handling ok ? - yes, the handling is excellent, bear in mind it is a big car though, if you are coming from an Elise it will feel a smidge blunt and heavy. If you are coming from a estate it will be wonderful! Lots of feedback and very controlable at the limit if thats your thing