If you get a carbon fibre frame bike for £500, it'll likely be knock-off, substandard spec, or both. There's not much difference in weight between most CF frames and the various metal ones. How light depends on your wallet. If you had £8k burning a hole in your bank balance, as low as 700g frame weight... And material doesn't make a difference on hills, weight does. As for ride quality, frame design & the lay-up of the carbon are important factors- there are carbon bikes that are rock-hard, others that are far more comfortable. Don't rule out metal frames or metal/carbon combos.