First you will need to get in touch with Honda and get a Doc from them with proof the year it was made etc you will need this to hand into the DVLA to reg it,
Then you will need an MOT which you can get a day time MOT so you don't need lights but you do need a horn and road legal tyres etc to pass it. If lights are fitted they all need to work. You don't need a speedo for the MOT but if you use it on the road and you get stopped the Police might have something to say about not having one fitted
Then you need to take i think it's a V55 form along to the DVLA with your insurance docs, the Doc from Honda and your MOT and give them some money then they will issue a reg number but they might also want to look at the bike too. Just make sure when you fill in the V55 where it asks for type approval just fill that in as Enduro so it's exempt from needing an SVA test
Then last thing you need to do is take your full bike test to be legal ride it
even then at 17 i think you might be limited to 33 BHP which i'm not 100% sure but i think the CRF250R might be over this but you could always do the easy thing and buy a CRF250X enduro model with the lights etc all fitted which will also be better geared for road use unlike the MX version.