Larger branches of Tesco, as well as Asda and just about every 'pound shop' or 'discount store', sell aerial extension cables. Sometimes they come as a complete kit, with (for example) the clips to fix the cable to the wall:
http://direct.asda.co...86374,default,pd.html
Alternatively you can buy co-ax cable, plus the relevant plugs etc, at any branch of Maplin (in most towns). That's handy if you want to make a specific length of cable but it can be a bit fiddly fitting the plugs onto the cable.
You'll note that the kit shown in the link includes a 'Y' splitter box, so that the signal can be sent to two different devices. Splitting a signal in that way obviously results in some signal loss along each cable. If you've got a good aerial, and you're fairly close to a transmitter, that might not affect reception. (Millions of homes have got that type of splitter in their aerial systems, with perfect reception at the end of each cable). However if the signal loss is such that you can't get reliable reception on both TVs you'd need to replace the Y-splitter with a two-way amplifier, such as this one:
http://www.maplin.co....gle&U=223503&T=Module
Chris