Need to buy ticket in advance to the Roman Bath ?

According to the RS guide to Great Britain, buying online means you get to avail yourself of "fast track" entry, so if you are going on a Saturday or Sunday this would save you some time standing in line. In October on a weekday there probably won't be a very long line. One comment on TripAdvisor implies there's a small monetary savings if you buy online, but the RS guide doesn't mention this.

An audio guide is included in the entry fee and there are also tours (with a live guide) at no extra charge several times a day. RS recommends the latter "for those with a big appetite for Roman history." The opening hours in October are 9 AM to 6 PM, last entry one hour before closing.

Here is a suggested itinerary for a one-day visit to Bath. It includes 10 places, which I personally would not try to do all in one day, but it may help you decide what -- if anything -- you want to do besides touring the Roman baths.

https://www.tripadvisor.com/Guide-g186370-k4542-Bath_Somerset_England.html

I’d just buy it at the time. Are you spending the night before in Bath? Just asking as there was no line when I visited in early June right after they opened.

hi
i will only be there for few hrs from london .

We did three nights in Bath and didn't buy in advance. It was in early October and not crowded.

No. We were there during the Jane Austin "Festival" and did not experience a line at the Roman Baths. I did buy tickets for a walking tour but I don't think this was necessary

We were there on a Sunday morning in June. No line.

When I was there in early July, there was a line in the afternoon, but I waited to go after evening service at the Abbey (around 6 or 6:30 maybe when I went to the baths) and walked right in. I had been many,many years ago and the audioguide and displays are much improved and I enjoyed my tour immensely. Be sure to listen to some of the Bill Bryson tracks on on the audioguide. If you are not familiar with him he is an American and Anglophile.)