To begin with these are most likely Galanthus nivalis which is the most commonly seen 'wild' snowdrop in Britain. They are probably not native but have been in the country for so long that they are now accepted as part of our flora.
How they got there in the first place is simple, someone at sometime will have planted a few or even a lot. From then on it is down to nature. G. nivalis does set seed so it spreads like that. Go out on a really warm sunny Winter's day and you may even see the mining type bees at work on them. A magical sight.
Also the bulbs split into 2,3 or even as any as 5 new bulbs so they soon build up into decent size clumps. These bulbs are then moved around by moles (in my garden at least) and any other soil movement.

spot on answer owdboggy. any critter scratting around will move the seed too.
I love snowdrops.

Welcome to GoY too.

I have them in the garden, they are glorious when they're out, didn't know about the bees Owdboggy, will keep an eye out

Sounds very nice searchfiled. If you haven't seen the house in spring or summer, there maybe some interesting things waiting to show. As difficult as it maybe, perhaps leave it for a year (!) to see. Never know, maybe some gems.
A friend who mpoved last autumn has now got an entire garden full of Muscari. Granted the garden was left 'abandoned' for 20 years by previous owner. Funny thing is, she hates the things! Amazing thou, they go down deep and have tiny bulbs that come up every where. I think i know what my job will be over the next few weeks!

We had a garden once where snowdrops started appearing a few years after we moved in - not planted by me. We thought they were a present from the birds as they first appeared under a willow tree and spread outward from there.