In England Scotland and Wales buses and coaches must be aware of the regulations stipulated in the PSVAR, which stands for Public Service Vehicle Accessibility Regulations. It contains a set of rules that is aimed at improving the accessibility of buses.
All full size single deck buses over 7.5 tonnes need to comply to the PSVAR already on the first of January 2016, and all double deck buses from 1 January 2017. One of the regulations in the PSVAR states that reasonable assistance needs to be given to disabled people to enter and leave the bus. If companies do not apply to the rules at the starting date they risk a criminal offence.
It is important for companies not to underestimate the work it can take to comply to all of the rules. Some of the buses that were build before 2000 might even need modifications done in order to comply.
On the UK government website you can read the full guidelines and more extended information.