I have an '03 XR250 and used to own a '97 XR400. These are US spec dirt bikes. Very similar, but engine and chassis different.
XR250 Much Much better off road. Both will require rear subframe beefing up
for RTW use. Perhaps you get a different model in the UK? No idea.
Both super reliable, economical and very capable off road. The 250 being far easier to manage in tough conditions (sand, mud, rocks et al)
They do share a lot of parts, as you've gleaned.
XR400: Bike came out in 1996. Was updated in 1997 and has not been changed since.
XR250: Been around forever and but a whole new bike came along in 1996.
No changes since then.
Both bikes now discontinued in the USA. (parts will be available for a least a decade)
Do the UK spec bikes have a rubber damper cush drive? Our US spec ones
do not. Over time on the pavement this can affect the transmission and even
the bottom end of the motor. The trans will begin to "whine" after a few thousand hours running on pavement. No such issues if ridden only on dirt, gravel, sand. Frequent oil changes help on this.
Both very reliable. 250 a bit easier to work on. Super simple. XR400 has oil cooler and needs it due to more HP. The 250 only makes about 16 hp and will run on any gas. Both mine have done a lot of Baja, Mojave and Sierra. Not one problem.
I have a lowering link on my XR250 that lowered it down 2 inches from stock.
I am 5'6" and its perfect. I lowered the front a bit by raising fork tubes in triple clamps. One of the best handling trail bikes I've ever owned and will
cruise on pavement at 70 mph. (six speed, wide ratio trans).
Open up air box, fit lighter, freer breathing pipe, re-jet and it will run forever,
and these mods will give about 3 more hp and better fuel economy. My XR is now fitted with a Clarke 3.5 gallon tank. Good for 200 miles on Mexican gas.
Few pics on the XR250 in its element.