All service areas are required to be fully accessible to the disabled. This includes providing standard accessible toilet facilities, but does not require them to have a Changing Places toilet. In 2019, the Department for Transport provided funding to increase the provision of Changing Places facilities.
Sometimes the disabled toilet will be locked to prevent miss-use, with staff able to access the key. Operators should be aware that not every disability is visible. The standards do not dictate the position of the toilets, other than that they should not be up stairs.
Specifically, they should be providing at least two disabled toilets in both the male and female sections, and at least one outside of the gendered sections.
At least two disabled parking spaces must be provided, "adjacent to the front entrance". Two more should be provided for the hotel.
Some service area buildings date back to 1960, a time when accessibility was not considered and steps were frequently used at the main entrance and to access second storey areas. Where lifts were provided, it was for convenience rather than any form of consideration. This attitude didn't really change until the 1980s, and that was probably more for commercial reasons (customers are less likely to head upstairs).
While most of these old designs have now been adapted - certainly as far as providing basic facilities is concerned - there are still some specific shops that can only be accessed via steps. Cardiff West, built in 1991, has a separate entrance and car park for disabled users because the main entrance has steps.
A symbol denoting that the service area is accessible to the disabled was used on road signs from 1982 until 2008, by which point it was being used on all signs.
Questions about disabled accessibility should be directed to the operator, or if it's about the UK regulations, to National Highways (in England) and/or their equivalents. The website AccessAble is useful for answering accessibility queries.