Yes - avocados can be grown in the UK.
It might not be the perfect climate, and you wouldn't be able to grow them on a commercial scale (not to my knowledge anyway!) - but if you wanted to grow a few lone avocado trees, in a sheltered and warm spot, then you should be fine.
There are 2 very famous avocado trees based in London which are quite old (probably over 20 years) which bear quite a lot of fruit!
It looks like these may have been grown from discarded Hass avocado stones, probably from a supermarket purchased avocado (although realising there are 2 growing together, I am a bit doubtful if they weren't pre-grafted trees...)
Growing your own avocado tree from a stone is a great way to grow an avocado tree, but if you want it to bear fruit, you are far better to graft on a cutting from a tree that you know bears fruit.
This is because otherwise you may be waiting 10+ years to find out if the tree bears fruit, and whether that fruit is tasty or not! It has been described as playing the "avocado lottery" - it is quite fun to do, but you need to wait a long time and you may never get to grow avocados.
Because you're growing in the UK where the climate isn't perfect, lots of factors might work against you. For that reason if you are serious about getting fruit then you should try grafting on a cutting (also called a Scion) of a desirable avocado variety.
A great plant/tree identification app which is also a nice example of citizen science which I rely on quite a bit is called PlantNet.
If you access this app, and look up avocado trees (Persea americana) and then go to their handy map view, you can find all the recording sightings within a specific location - in this case the UK.
As you can see from the above - there have been more than a few recorded sightings of avocado trees growing in the UK.
Obviously I can't verify or vouch for these, but if you really wanted to you could probably trawl through the photos found, and zone in on the specific locations, to understand more. Perhaps these are being grown within nurseries, greenhouses or nice microclimates.
From the map of the UK we can see the majority of the sightings are in the south of England, which makes sense, as the climate there is warmer than up north.
Surprisingly there is an avocado tree growing in Scotland apparently - but again it would very likely be grown in a polytunnel or greenhouse of some kind - as despite being quite an optimistic grower, I'm doubtful that you could get an avocado tree to fruit and survive that far north, without some special assistance!
This project I am running (Aguacateros) is based upon building a network of avocado growers around Europe (and eventually the world!).
As part of this movement, we urgently need to find avocado trees in the UK, and elsewhere, whereby the owner of the tree is happy to participate in our network.
This means, they are happy to take cuttings of their tree, and to pass it on to nearby avocado tree growers - people who might have grown a tree, but they want to graft on a cutting (or scion) from a fruiting tree.
Because our project is in the early stages, we really need to try and build connections with people who have successfully grown avocado trees in the UK.
I am based in north Spain (but am from Bournemouth, Dorset, originally) and so sending out a cutting from one of my trees is difficult, not just due to postage costs but also as legally I think there are issues if a plant passport isn't provided. As I'm not a registered garden nursery, nor am I doing this for commercial purposes, it's not quite clear if I can send out cuttings. I'm also not convinced they would survive the trip!
So - if anyone knows of successful avocado growers in the UK, who might want to participate in our growers network - send them our way please!
We are always happy to add people to our network - whether you want to get access to an avocado tree, a cutting, or to contribute to the movement.
You can join our Signal group, or you can simply reach out via email.
Note - we're not currently active on social media - but if you can share our project over there, it would be greatly appreciated!