Most of these Swindon companies are much of a muchness. I believe Prestige offer the best value and it's what my sister uses and thinks is very good.
https://www.prestigesuperfast.co.ukEdit: I can see Relish is better value.
On a side note, and Reg would like this one, but I know of a bloke who works for openreach who said most the rural outskirts of Swindon will NEVER have fibre as the council will only fund the installation of 4G. Wankers.