One is a bronze age founded town, and the other started life as a saxon settlement, both have no bearing on new towns, or their expansions post war.
All towns and citys in the UK have expanded, its called progress. London was timber surrounded fort, then the Romans came along..the rest is history.
I know that <cough> sort of, anyway..........I was in Swindon when it kicked off in the 70's and knew a female member of the family who owned, or had options, on most of the land between Swindon and Wootton Bassett*note 1. I wasn't shagging her, a mate was, I was busy with her cousin. Swindon indeed has a long history and became an industrial town when Isambard built his railway works there, later Plessey, and the British motor industry moved in. However, in the 70's the local Thamesdown Borough Council, at the time, started referring to it as a new town. Effectively the original Swindon was used as an anchor to develop West Swindon, known at the time as Toothill*note 2, and then subsequently North Swindon on the road to Highworth.
Likewise, you mentioned Milton Keynes, the long established villages around MK were used as the anchor for its construction. MK was my local shopping centre for a few years before moving here, and I was coppering in an adjacent county so I observed some of the original villages which had been swallowed up by its construction.
Most new towns used existing villages as their anchors when they were constructed, if only to build dual carriageways between two villages thus providing easier builder access. Back in the day when West Swindon started to go up and the imports from London and Birmingham started arriving the local Swindonians considered it another town entirely.
Notes:
1. The land between Swindon and Wootton Bassett became West Swindon, and the family firm made squillions.
2. Toothill was thus named as it was a low'ish, long hill that the railway tracks ran up from the direction of Chippenham/Bristol and the steam trains would toot their horn when going up the hill in case there was something/someone on the other side. Long time proper locals of Swindon still call West Swindon by the name Toothill.