Welcome to TBN Mith. I see you've posted a few times, but welcome anyway.
Are you speaking of PTO HP, or gross engine HP? What you need to be concerned with is the PTO HP as that's what's going to power the mower.
You have two things to think about, weight of the implement and power requirement. Most light duty mowers will do fine with 15 PTO HP. This generally will mean you're limited to a 4 foot mower. I use one on my CK20 with 15.5 PTO HP, but I can tell it would run a 5' in light brush. Mowing is one of the most taxing on HP. The larger the mower, the more HP required, and the heavier they'll be. You will most likely need some type of front ballast as even the smaller mowers weigh a lot.
As for NON-PTO implements like a box blade, 48" again is about the limit, and some MFG offer "estate" sizes that may be better suited. When you're ground engaging, there is a lot of resistance and it can pull your tractor down quickly. However, with a little creativity, like removing a couple of the rippers, digging less deeply, etc. you can get by without much trouble. I'm assuming a lot since I don't know what tractor you'll be using, but for the HP you mention, most things are going to be in the 36" to, at most, 60" sizes depending on the above. General engine HP in the range you mention is going to be very little PTO HP. Most of the tractors that come into the US from Japan are rated in their model numbers with PTO HP. I don't know about the ones in the UK. John