Motorway Industry Regulator
Since motorway services were deregulated in 1992, the industry is supposed to be a totally free market. There is no industry regulator, because the government theory was that competition would be enough to keep standards high.
In 2017, the independent watchdog group Transport Focus started an annual review of motorway services in England. They cannot force through changes (and don't take individual customer feedback), and services can opt out of their reviews.
Local authorities have powers over some issues, such as environmental health and planning law.
Highways England are responsible for setting out the regulations that govern services in England, which are usually adopted by highway authorities across the UK. If a service area breaches those regulations, it should have its road signs taken down and its connection to any motorways sealed off. However, these regulations only deal with technical matters (like the number of parking spaces), and don't cover the standards of facilities.
In 2008 it looked like things might change. A new document called Circular 01/2008 made the regulations much easier to read, and contained a reminder that operators should "endeavour to provide high-quality facilities". It also called for an independent rating system. In the introduction, it stated that the Highways Agency (now Highways England) would only intervene with breaches of policy, and not "other operational matters" - it then went on to specifically ruled out responsibility for pricing or staffing complaints.
This turned out to be a false alarm. New government policy in 2013 called for the industry to be de-regulated again, so a new document was produced that removed those few references to quality - and most of the other regulations. It also removed the rule stopping services being built, which is what the Highways Agency used to use to discourage poor quality proposals.
Prior to 1992 (and especially prior to 1979), motorway service areas had to work closely with the government, who owned the land most of them were built on. Responsibility passed through several departments, most memorably the MSA Board. While they didn't have many powers, they had a close relationship with local managers which they were able to use to encourage improvements.
Who owns the land?
Aside from 21 exceptions, all service stations are built on privately land. Even in the case of the 21 owned by Highways England, their agreement with the operator does not give Highways England any additional reasons to intervene.