Personally I would focus on butterflies that are actually native to the UK, and I would advise against buying non-native species online and deliberately releasing them into the wild. We know how that sort of thing can end up.
But Asclepias is a good pollinator plant generally as well, so definitely one to seek out and grow.
There are still monarchs in the British isles and I looked up on the internet about it and it said that they were very common in the UK. People want to see more of them so i would encourage people to plant milkweed in order to make a bigger population of monarchs in the UK.
Butterfly Conservation record it as a very rare immigrant to the UK, most sightings in Cornwall and the Scilly Isles. Not known to breed here because of the absence of its food plants.
The chance of one finding the Asclepias you plant in your garden must be close to infinitesimal, but there's nothing to stop you trying
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But Asclepias is a good pollinator plant generally as well, so definitely one to seek out and grow.
Maybe grow some plants to attract them too?
People want to see more of them so i would encourage people to plant milkweed in order to make a bigger population of monarchs in the UK.