Hi trj977,
This is an area I know fairly well and cover about 3-4 times a week. So yes, I would do the A308 route to Staines too, I dont have a problem with it. Sunbury Cross round-a-bout is okay if your okay with round-a-bouts, but there is a fully cycleable subway there if you dont want to cycle it. Once you get to the fordbridge round-a-bout, I wouldnt go on the Staines By-pass, I never have, cycling that is. Instead take the first left then next right to take you along Kingston Road into Staines. Then you have two routes. One is the Stanwell Moor road to the colnbrook by-pass which you can then follow straight into slough. Not Very scenic and half way along the Stanwell moor road, there are cracks across the road every 5metres or so for about a mile, what makes the journey annoying, bumpy and stressfull. But, the route does take you past Spearmint Rhinos, if you into that sort of thing! The other way i would go would be to carry on the A308 into Old Windsor, B3021 to Datchet then up to you which road into Sluff.. Both routes, I would say are fairly well abused by speeding cars, but that nothing unusual around here!
Good luck! Let me know if you need any more help!