Business UK was introduced as a competitive counter measure to BD Domestic Business Class.

Some years ago BD flew a small C cabin on domestic routes in addition to other international shorthaul. This put BA under some pressure as a BD C fare could be less than a bog standard UK domestic. The BD C fare also came with lounge access and full meal (a hot breakfast even on LHR-MAN).

This prompted a number of reactions from BA including the introduction of the breakfast baguette on the short domestic routes (which didn't have time for a full service) and "business UK fares" which gave lounge access on fuller fare domestic fares to those who had no BA status.

It is just a regular domestic service, the business UK tag is simply there to highlight to lounge agents that the ticket qualifies for lounge access.