OK let's clarify this. Three percent and six percent hydrogen peroxide solution remain on what's known as the General Sale List in the UK, which means there is absolutely no reason why you should not be able to buy it in selected supermarkets, poundshops, discount stores such as B&M Bargains and of course, pharmacies. The GSL status of low concentration hydrogen peroxide is taken from the current edition of the BNF, which is essentially the "Prescribers Bible".
If you are having problems purchasing it in shops, this is almost certainly due to the shops not wanting to stock it. They are entitled to do so if they wish but please take with a pinch of salt any statement they may tell you about not being allowed to stock it. Hydrogen Peroxide has been used for some nefarious purposes in the past and it is right that a store has the ability to err on the side of caution. I understand that some pharmacies are also up to the same tricks by also asserting they are not allowed to sell it hence discouraging hydrogen peroxide sales.
Hydrogen peroxide is also still on the dental practitioners formulary, which means that dentists can freely prescribe it for use as an oral mouthwash subject to dilution by the patient. Those who have had it prescribed by a dentist should have no difficulty collecting the stuff from any pharmacy in the land on presenting the FP10D prescription form.
As others have found, hydrogen peroxide is freely available on the internet from online pharmacies. If low concentration hydrogen peroxide solution was restricted in any way, such sales would be forbidden.
Remember, hydrogen peroxide has a very low retail sale price and profits are likely to be minimal on sales. This may play a part for some retailers.