L2B - considering buying a road bike

[jumplead]

So up until now I've been doing what little training on a 1998 vintage fixed frame mountain bike (GT Palomar for those interested).

A work colleague who recently did a John O'Groats to Lands End ride offered to loan me his road bike to do the L2B on, but it appears that in the transport home the rear wheel has become buckled and may not be repaired in time.

So I'm thinking i'm just going to bike the bullet and buy myself a cheap road bike. But the question is, are the cheaper road bikes worth buying or would I just be better getting some slicks for the mountain bike? Also are these cheap bikes designed to be hauling my fat ass (18 stone) around?

Two that I've found in some very brief searching are:
http://www.evanscycles.com/products/claud-butler/c...
and
http://www.halfords.com/webapp/wcs/stores/servlet/...