Royal Mail doesn’t release pre-made maxis but you can make them yourself by combining the stamps with PHQs (or other relevant postcards) and sending them for postmarking. As far as getting the right postmark, I believe you take note of the corresponding number as listed in RM’s postmark bulletin and send it to the address specified. The British Isles sub-forum might be the best place to ask for more info, and you can read about requesting postmarks in this thread: Royal Mail Special Handstamps - how does it work?
Good luck for your new hobby 