not positive but I'd guess Pennines, Cairngorms, Snowdonia, Brecon Beacons/Cambrian Mountains, Southern Uplands, Highlands. Could be Wicklow if you include Ireland.
Experts generally agree that, in Britain, a mountain must be over 3,000 feet high. On that basis, I'd say that there simply aren't six such mountain areas.
The Cairngorms, the Grampians, the Western Highlands of Scotland, Snowdonia and the Lake District would just about cover the appropriate regions.
Not neccessarily- a mountain over 3000ft in Scotland is known as a 'Munro' - have bagged a few of them! There are also several 3000 footers in Wales and the Lake District. A mountain of between 2500 and 2999ft is a Corbett- some 219 of them in Scotland and 17 in Wales and England. A Graham is classed as a mountain between 2000 and 2499 ft. So you can include the Pennines in your mountain areas.
You're absolutely right, Burnhal. Complete memory-slippage on my part there...the height for a British mountain as opposed to a hill is 2000 feet, not 3000.