The WR250R is a good choice but expensive for a 250. I assume you can't afford a new one? Used ones also rare here in USA. Most are clapped out by the time they show up for sale used.
I would look closely at the Honda CRF250L. Great travel bike. I've no idea if used ones are available in UK. Here, they are priced well new.
Figure about 500 quid minimum to prep for travel on any bike you find.
If you're flush, consider the CCM 450. Around 8K ukp.
If on a serious budget also consider:
Kawi KLX250S (great with a 300cc kit added)
Suzuki DR350 (road legal version)
Suzuki DRZ400E or S (very popular)
Honda XR250 or XR400 (kick start only)
Yamaha XT250
I would avoid any KTM for long distance travel. Unless you are an experienced mechanic and know KTM's well ... and don't mind carrying a complete parts inventory on board. Also, way overpriced $$$$ IMO. Not good value.
BUT ...
I would recommend buying a two stroke KTM for learning off road before you depart. The 200EXC is the perfect novice learner off road bike. Amazing bike, but NOT a travel bike. You're learning curve will go straight up quickly starting on this bike or one similar. LIGHT IS RIGHT! Two Strokes rule off road!
All bikes listed need prep for comfort and luggage carrying ability. 250's are tiny bikes, so a challenge to pack a load onto. IMO, a good seat is primary to handle 12 hour riding days you may encounter heading towards Mongolia.
I've owned the KLX, DRZ400E, DR350 and WR250F (not the R), and for me, none were good LD travel bikes. All great off road of course but unless trail riding, they were limited. Not good all day bikes, slow on fast paved roads. After 20 years on many different dual sports I returned to the trusty Suzuki DR650. (not for sale in UK, but can be found)
Why DR650?
Wide seat fits it, great carrying luggage, simple and light weight (147 kg. dry), decent economy, tough as hell - stone reliable ... easy to ride and easy to work on yourself.
Once set up nicely it's quite handy off road, even fully loaded. NO, not a single track Trials bike or Enduro Dakar bike, but perfect for most rough two track and open ground at speed... and can cruise the motorway comfortably at 75 mph ALL DAY.
65,000 miles on mine. NO PROBLEMS!
