Given your location I could highly recommend Ady Smith's off road training days, he will provide you with a KTM, the wisdom of years as a winning international enduro rider and at a lot less cash than BMW, but.... an Enduro bike is not a Big Trail bike, and a tight woods enduro is not comparable to riding rough roads.
I'd go green laning, maybe you could buy a little trail bike and sell it on so apart from capital outlay you shouldnt be too out of pocket, you'll get something fine for £1500 and so even if you sold it on for £1000 6 months later youre laughing
I've been on a few forum ride outs with the ktmforum, advrider and UKGS'er and even here, they have all had either a novice group or were specifically for heavy bikes with maybe less suitable tyres, all were good fun if sometimes quite slow and I have only seen one bike really come a cropper, an SLR650 and he could have gone round the waterhole
nevertheless we got him going.
if you arent pushing yourself too hard, respect your own limits, make sensible modifications such as crashbars, handguards that you'll want for your adventure then your bike shouldnt have any serious damage after a green lane outing providing you take to lanes that arent too mad.
A weekend down salisbury plain way could be good though that chalk can be slippery when wet! Wales is ace and there are some companies advertising in magazines like TBM that have dirt bike friendly B&B's, know the local area, and would probably mark you a route or guide you on something suitable.
enjoy I do
KTM 990 adv on the motorcross track - YouTube