Leaf vegetables should be reasonably safe, because we've bred the toxic compounds out of them. I haven't tried it, but Apple, Hazel or Elm leaves should also be shrimp safe. The used to cut Elm as <"tree hay"> and feed it to the cattle in winter.
Most plants, with similar nutrient rich composition, protect themselves with a whole raft of fairly toxic compounds (alkaloids, oxalic acid, latex etc), but nettles don't, they rely on their stinging hairs.
I feed the tips of nettles to my shrimp and give them a quick blanch to help make them easier to eat. I've also tried chard but they didn't seen overly keep compared to the nettles. Dandelion leaves are a good source of nutrients and i blanch them as well. One of the best foods is nettle pollen and I collect and dry it to feed over winter. Other flowers like from runner beans are also good and you probably have more choice with an allotment.
For longer lasting food oak, beech and hornbeam leaves seem to be eaten plus common fruit leaves like Apple, pear, plum or cherry. Small pieces of these woods are also safe.
You can make a simple food to be frozen from a 50-50 mix of nettle and dandelion leaves mixed with a little Spirulina and gelatin.
My wild meadow (or as the misses calls it, back garden in need of a big sort) has a little patch of stingers.
My only concern about using them is pollution. I live in Birmingham (albeit about as far out as you can get and much closer to farmland than industry) and live on a relatively busy road (but these are round the back and at least 20m from the road)
Would these still be OK after a little blanching or am I better picking some from a more rural setting?
I was taught that air pollution can be deposited several miles from the source (up to 40km to be exact) and that pollution is often far worse in the countryside. Since lead in fuel was phased out, studies have shown that roadside vegetation is far less toxic. The main risk would be the illegal burning of waste nearby. If I was you then I would go straight ahead and use them. But you actually have a reason to visit the countryside and spend some time collecting nettles, so why not bop out for a spin