The path to Bath is a very good start (except when it's full of skateboarders doing a charity ride!). I'd probably then head up Bannerdown, past Colerne, north of Chippenham to Sutton Benger, Broad Hinton, into Wroughton on the A4361 (maybe) and stay south of Swindon before heading on to Wantage, through the Lockinges and Harwell, skirting the southeast corner of Didcot – the gravel path along the railway to South Moreton has been pretty good the couple of times I've used it, doesn't look like it would get too muddy even in winter. And after that, I have no idea.
Disclaimer: Although I've ridden all those roads, I haven't put them all together into one route in exactly that way (specifically around Swindon, I often go north instead0, and not necessarily as an audax either.