Mainly Front Wheel Drive, though the BMW competing, and the yet be announced officially, Xero Lexus, are both rear wheel drive.
Just to confuse matters, there are also currently two different specifications competing. Those being British Touring Spec, unique to the UK, the Hondas, Astra and WSR MGs are built under these guidelines, as are some of the independant entries too.
The other spec, is Super2000, which is the specification as set out by the FIA (the international motorsport goverening body, ie Formula 1), the SEAT Leon's are built to this specification, as well as a few other cars including the independant ex works Schnitzer BMW of Martyn Bell for example.
The BTC spec cars tend to have an advantage over S2000 spec, so because of this the S2000 are granted a weight break in the rules and regulations of the British Touring Car Championship, so they able to compete on a level playing field, this is monitored by the BTCC adminstration throughout the year, and they do have the power to change the regs.
This in theory gives a more balanced racing environment, there are also weight differences given between rear wheel drives and front wheel drives to, and differences for the gearboxes used, cars using H pattern gear boxes get a helping hand, over the more competitive cars using sequential boxes.
The BTCC is unique in that it allows at the moment both types of spec to compete and win the championship outright, but from 2007 they are changing the rules, to only allow S2000 spec cars the ability to win the championship outright. Independants will still be able to run with BTC spec, but won't be able to win outright championship victories, as they won't be elliagiable for the points.
Its an effort by the BTCC to attract more manufacturers back to the British Championship, and by using the same rules as the successful FIA Worlds series, they aim to bring some of the big hitters back, like Ford, BMW, Alfa etc in the future.
More info on the regulations are here on the Official BTCC website.
http://www.btcc.net/html/regulations.php
Also the Full regs, (not light reading)
are here.
http://www.btcc.net/pdf/regulations.pdf
