The "parts of British Steel" are described elsewhere as: "The offer is understood to include British Steel’s rail service centre business, responsible for the welding, finishing and storing of rails for the UK▸ ’s rail network, according to a story first reported in the Sunday Telegraph."
So not the rail production business, certainly. It looks like something that only has NR» as its end customer, and might have been done in house anyway (if fashions in such things were different).
I've just found out a bit more about BS's Rail Technical Services division, and should correct that. They have a rolling mill at Scunthorpe and do produce steel rail. It is in effect part of the steelworks site, and starts with billets (or whatever they are called) from within the site. They will also have other big machines for straightening and other finishing processes. There was another site at Eastleigh, owned by NR, obviously without on-site production.
They were rolling steel to 108 m lengths, for supply as is or flash butt welding pairwise into the 216 m lengths that are the longest NR can transport. In March 2018 they got a big order for 120 m rail to Germany, having painted go farther stripes on their mill. That division also does work in the the applications areas of coatings, noise reduction, repair and rejuvenation, etc.