There are a few choices:
Urea Hydrogen Peroxide drops work for me, though it takes a few applications over a number of hours and it can initially cause a temporary blockage in the ear due to expansion of the wax as it breaks down. Over the counter in any supermarket pharmacy.. I think I paid £4. They work by converting hardened wax into a foamy sludge (with gas bubbles in it) which you can flush out with lukewarm water (multiple times). Don't use this product if you've scratched your ear with your fingernail, as it will sting.
Alternatively, vinegar ear drops seem to work (for me). They're branded as EarCalm at the high street pharmacy near me, and are administered via a dosing spray; I expect other brands are available. They're actually sold for minor ear infections. I think I paid £7. Far less dramatic that Urea Hydrogen Peroxide, this spray liquifies the wax and allows it to run out of the ear where it can be wiped away with a tissue (for me).
In each case, it's most effective to stay lying on your side with the affected ear uppermost for 5 minutes after application, to let gravity transport the liquid down to where it needs to be.