My first guess would be a screech owl, though I don't think you get them in the UK iirc. Similar to a barred owl, but again not sure you get them in the UK either.
Where do you live?
It's quite hard to tell, as many owls look quite similar and that one is evidently quite young and thus won't have formed the distinctive feathering of some species. It's not a snowy owl, that's for sure

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Edit -
Probably a horned/eagle owl thinking about it, how big is it? Actually, more that I think about it... It isn't likely to be an eagle owl as there are so few. You really ought to phone the RSPB straight away though.