I'm surprised Warmley is cheaper than Bath. Longwell Green, Keynsham maybe, but Warmley?
Anyway, from what I have seen of of the place I like it. Beware though, the council want to build extensively on green belt land in that area:
http://www.saveourgreenspaces.org/swag_index.html
...you don't want to buy a nice place with a view only to find a couple of years down the line they slap a housing association blot on it.
As to commuting, you'll have to offset what you save by what it costs to commute, both finacially and emotionally. I moved to Keynsham for the very same reasons to commute to Bristol and sometimes I wish I had just bought a small place near the centre instead. Despite being geographically close it can take an hour each way due to general congestion. And Firstbus are rip-off merchants-you'll be looking at circa £4.00 each every day, even on the P&R.
Parking is equally expensive. NCP is about £8.00 per day. I think it's about the same in Bath e.g. Charlotte Street.
Best of luck.