OK. Thanks. But I am staying in bath. Would it be possible to get a bus from Bath to Bristol for a day? If yes, I would look at options in Bristol also.

You can use the bus but it'll eat into your time and you're not missing much en-route.
If you're staying close to Bath station, the train takes around 10-12 minutes if you catch one of the trains from London, as it's non-stop from Bath to Bristol. The local trains aren't much quicker than the bus service, so avoid them.
From memory they used to run every half an hour until around 11pm.
The buses also run from opposite(ish) the station, should you feel a pang of guilt. The only advantage to the bus is that they will take you into the centre of Bristol, but on a side where you won't find many SPs - all shops.
The train will take you into Bristol Temple Meads, which is about 15 minutes walk from Bristol city centre and also 15 minutes walk from Harbourside, where a few SPs work from, or a short 5-7 minute taxi ride. If you walk it, go out of the side entrance of 'Temple Meads. If you go for a taxi, they're lined up out of the front of the station, night and day.
There's a few parlours in Bath, all Thai from memory, various reviews on here.
But for choice the best bet is Bristol as stated by Rudolph.
