Rare Butterfly?

Looks more like a moth to me, some kind of Ermine. Hard to tell from pic.

Edit, scratch that, think I’ll also agree with small blue

Looks like a Small Blue. Not hugely rare but depends on where you are I guess. Think they're more southern. *edit* not the greatest of pics so Houns' call of Ermine could be correct, he probably sees way more of that stuff than I do.

Is it not a Holly Blue?
https://www.dorsetbutterflies.com/species/holly-blue/

I'm near Ipswich in Suffolk (unfortunately), but I think it's a Holly Blue as MrSparkle suggests.

And apologies for the pic quality, it's quite clear on my phone!

I think it’s a Holly Blue - that’s going by the small number of dark spots on the underwings, most blues have lots more, plus some have quite a lot more general colouring as well.
Butterflies are bloody difficult to photograph with a phone, they’re notoriously twitchy and won’t hang around; pretty good job getting that pic!
I went over to a nature reserve last week, it’s just outside Calne, on high chalk grassland, so it’s good for orchids and various butterflies, moths, etc. A new information board had been set up, and it featured a butterfly I’d never seen before, so I thought I’d keep my eyes peeled just in case. Ten minutes later I spotted one, then quite a few chasing each other around. This is the first one I saw:

It’s a Marsh Fritillary, although what they’re doing on dry chalky soil 250m above sea level I have no idea. Although there are Marsh Heleborine orchids there in their hundreds, the only downland site in the U.K., AFAIK.

Go figure, as they say.

Just done a search, and I feel highly privileged at having seen not just one, but at least half a dozen. They were all recent hatchings, their colours fade very quickly, and they are threatened right across Europe and the subject of majore conservation efforts.

https://www.wildlifetrusts.org/wildlife-explorer/invertebrates/butterflies/marsh-fritillary