OP, I've sent to addresses that have no number before in the UK, and they've always reached the destination. However, you would need a house number for a public street. The times I've sent to addresses with no number were for agencies that DO NOT have numbers on their website. Therefore, it was always bound to reach them, because thats how they advertising the address on their own website.
With someones house, you're going to need the number, else the postman isnt going to know which house he's delivering to... and while the UK postal service use the postcode more than the actual number of the house, its better to include EVERYTHING so that it will definitely reach the targetted person
