HMRC have two tests to decide if an expense is tax deductible
The expenses must be “wholly, exclusively and necessary” for the business and there must be no “duality of purpose”
This means that if you can have a personal use of an item (even if you don’t use it personally) then it is not fully business and so no tax deduction
HMRC use a case Mallalieu v Drummond to support their argument which almost always go their way apart from very unusual and exceptional circumstances in the case of G Daniels v HMRC
Under self assessment unless you have a tax enquiry then the expenses you claim are not questioned, if however this happens and there is no agreement with HMRC the case goes before a tax tribunal who the make a decision based on the facts and arguments presented. It is open to interpretation, if you lose HMRC can then re calculate the profits for up to 6 years and there is then tax , national insurance interests and penalties to pay
It means that, in my opinion, you cannot generally claim for such as these for example
Glasses for work (or eye tests), unless they were protective glasses for safety
Dental treatment
Nails or other cosmetic treatments
Work clothes unless they are protective clothing, a uniform or in the case G Daniels, dancers outfits
Personal therapy for Psychotherapists and counsellors, but supervision would be allowable
Cosmetic surgery
