Hm, missed that bit! Not sure what shorts are on the narrower cut tbh…
Jeans have a crucial problem in the UK outdoors which is that when they get wet, they stay wet which obvs leads to chilling, chafing and so on.
Depending on your waist size, maybe larger youths’ sizes might work? Alternatively possibly check European stores such as Alltricks.fr, Bike24, Bikediscount.de etc. British mountain bike clothing sizes do tend toward the ‘more generous’, possibly overcompensating because there have been so many complaints about it being offered in ‘euro-roadie sizing’ at times from some brands…
Alternatively, maybe look at general outdoor trousers with a view to cutting down… once you have a set of padded bibs then considerations of seam placement etc become a lot less important, and freedom of movement, durability, water resistance, breathability etc become the key points – all of which should be covered by any self respecting outdoor clothing.