For Road reg in UK you need....
NOVA reference from HMRC to confirm no VAT due (1-3weeks)
MOT
Insurance on Chassis/Frame number
Once you have all the above, fill in a DVLA V55/5 form and send it off, make sure you list it as "Enduro" as exemption from type approval.
My XC250 2011 came with Cert of newness from GGUK (?25), I filled DVLA form in as EC (frame number VTREC....) and it went through without a hitch, probably took 2-3weeks after I got it MOT'd and sent everything off. I done this last year, here in Northern Ireland we don't get English plates so mine came back with a 2017 reg
Have 2 other bikes im doing for friends at the min,
1 is a 2003 EC300 the other a 2005 EC200
GasGas UK have no record of them and are unwilling to (officially) confirm the year (10th digit of VIN).
GGUK have told me GGSpain are absolutely useless at this sort of thing, basically unless you have similar query with a new bike they aren't interested in supplying data for old bikes.
Not sure how these 2 are going to go because we have no proof of age, History etc, so could end up on a Q plate that can be difficult to insure.
Have managed to get a NOVA reference for 1 of them though