Long weekends away.... I guess you mean, all day Friday, Saturday, Sunday, back home Monday night?
Does this embrace the however long it is getting from your Scottish retreat into (mainland) Europe and back again?
Do you intend to fly / drive or go by boat / road?
Why not start with those closest to you:
Holland.... Ypres.... then the fields of the Somme......The great thing is that these sites are all close together. Then think about a bit further south to say, Verdun.
Then, look a bit further afield... The D Day beaches are miles away from Calais.... the U-Boat pens even further.....
Really, have you given this any thought at all? The major WW1 sites are not particularly close to what the Brit’s see as the ‘must do’ WW2 site of D Day.
Get yourself a half decent map and go to the library (it will save you buying anything) for Major Holt’s Battlefield Guides
http://www.guide-books.co.uk/index.html or something similar. Work out where the major sites are. Do some research of your own, rather than being spoon fed every idea which probably won’t suit you, as the well intentioned advice giving bod won’t have factored in you getting from Aberdeen to Arromanches and back without using up most of your very cheap long weekend away and you intend to walk.
PS Many of the guided tours are very good value, as the guides know how to pack things in. Yes, you have to pay but that is how it works; they earn their living organising something that bods don’t know how to do themselves or want someone else to do for them.... which is the purpose of your enquiry after all.... on top of which you want the accommodation, cheap presumably, too?
Try a look in the Travel section. You’ll get lots of ideas. You have paid £12, so it will be good value. If it isn’t complain.
PPS Aw, feck it. Just Google ‘self guided battlefield tours’ or something like that. The one to Gettysburg won’t suit you, though.