You could go to Boots/Superdrug and start Propecia/Finasteride, it more or less completely stops balding and sometimes reverses it (mostly in the crown), you'll need to see one of their 'doctors' who just look at your head and confirm you're starting to thin out.
It's pretty costly tho, can run around 25-30 quid a month from some places, you could also get a private prescription from your GP which may work out cheaper for the generic finasteride.
I've been on it a year now and not on any HRT.