Hi Twinkle,
There's 3 pubs within 5 minutes of Junction 15 (Swindon East) that I know of.
If you leave the M4 at J15 and turn left at the roundabout above the motorway you'll find a pub about 200 yards up the hill on the right (can't remember the name I'm afraid, but I was told that this is where some of the planning for the Brinks Mat robbery took place!?!)
If you turn right at the roundabout and follow the signs for Princess Margaret Hospital you'll come to a second roundabout, turn left again and follow the dual carriageway for about a mile and you'll find 2 more pubs (one of which is called The Sun) on the left. There's another roundabout (this is Swindon after all!!) at the end of the road so you can retrace your route back to the M4.
These all serve pretty standard pub food, but won't delay your travels too much - a word of warning though - steer clear of the local beer (Arkells), taking in any amount of this during your meal will make for an interesting remainder of the journey with plenty of rushed, and potentially embarressing, stops.