In reply to midgen:
The problem the OP has is down to the size of their feet, a UK 3.5 Eu 36. Most shops within the UK don’t go below a 5, a few do go down to a 4, and I’ve only came across one that did a 3.5. However the original question was to do with trusted overseas dealers so here’s my take on what ones are imo good.
Trekinn: no extra charges at all, ordered a pair of boots at £250.
Deporvillage: no extra charges but I’ve never gone above the £135 customs threshold.
Addnature: no extra charges again never ordered above the £135.
Bergzeit: no extra charges never ordered above £135.
Amazon are a good bet as well. I’ve ordered a lot of stuff from there, which has came from the EU without additional charges.
For orders above £135, I would email any company I was going to use beforehand to clarify. Also check out the HRMC website as some goods are exempt from custom/import charges, but it is a complicated site.