If it is under 0.5cm then it could be too small to need a ramp, but anything over that needs either a slanted edge or a ramp to be considered level access. The equality act doesn’t specify when you might need a ramp, but it does say that disabled people should be able to access shops, businesses and public functions on an equal basis with non disabled people. Fortunately there is some guidance on the matter in building standards. The standards for ensuring access to premises for disabled people are BS 8300, Design of buildings and their approaches to meet the needs of disabled people – Code of practice and this suggests that any steps in new buildings built over 0.5cm should be chamferred and anything over 2cm must be ramped. Other building standards found here suggest anything over 0.5cm should be chamferred, and anything over 1.5cm should have a ramp provided to be accessible. Even if you aren’t in a new building you have a responsibility to make sure your business is accessible to disabled people, including people that are wheelchair users. This means if someone can’t get in over your step, however small, it would usually be reasonable to provide a ramp.
If you have a threshold rather than a full sized step you may find that a threshold ramp is more appropriate for your business than a full ramp. You can get one here from £18 including VAT if your step is up to 7cm.