It used to be possible to get fire-door 'blanks' that could be trimmed, but you'd need the manufacturer's information on what's allowed, if they're still around. Otherwise you'll need a custom made doorset.

Posted By: Mike1Otherwise you'll need a custom made doorset.

something like this:

https://www.madetomeasuredoorsuk.com/made-to-measure-fire-rated-doors

Just checked the Howdens site. They offer F30 and F60 solid core doors which may be cut to suit the opening.

Good to know that they're still around, however note that the entire doorset needs to comply, not just the door. Building regs Part B, says (the bold type is in the regs, not added by me):

C4 Any test evidence used to verify the fire resistance rating of a doorset or shutter should be checked to ensure both of the following.
a. It adequately demonstrates compliance.
b. It is applicable to the complete installed assembly. Small differences in detail may significantly affect the rating.

You therefore need to refer to the information from the manufacturer for the frame and fitting details, helpfully provided by Howdens at https://www.howdens.com/-/media/howdens/assets/clh_asset_products/clh_asset_levela_108973/ass_25308090.pdf