"Dumfries and Galloway, Solway Coast perhaps ? It's not as "Scotlandy" as the highlands, but lots of good MTB trails."
Whilst he could have a lovely holiday there - I suspect it might be seen as "his holiday" if she realises he is in "trail centre heaven"... not to worry there are plenty of real trails on real mountains if he takes her where she wants to go.
I could probably recommend some but your brief is too broad. Is it just for 2. Do you mind paying for a week even if only staying for 5 days. Is it in season or now? It also depends what you mean by remote/secluded. Some people would still expect to walk to a pub, whilst I would say if you ever see anybody else's car when at the cottage it's not secluded! I could suggest an island accessible only by causeway - but there are 3 other cottages so still not total isolation (and the cottages weren't that good!). I love remote/rural/secluded but actually my favourite spot is in a tiny village (one shop, one pub) at the end of a single track road. Its not that far off the "main tourist route" either - but the road is small enough that coach parties don't go there!
I think you need to see if she has a particular area in mind. e.g. if she was thinking Skye / Applecross after watching Monty Halls and you take her somewhere different she might be disapointed. You also need to narrow down your cottage spec. Are you looking for luxury in the middle of nowhere or a run down shack for the proper croft experience? If its October - April then is an open fire on dark evenings part of the appeal etc.
?? Part of the luggage innit?)
