As spudlet says, Cotswold Outdoor has 'proper' stuff, and Trespass should be OK also. As *Ifailed' says, look for a label saying how waterproof the jacket is. And I agree with her about hoods, though you can get waterproof hats and caps.
In the sales, look for brands such as Berghaus, Mountain Equipment, Lowe Alpine, Rab (though they tend to specialise in warmth, not waterproofing) or (Rolls Royce level) Marmot.
Regatta is a fairly low price brand but perfectly OK and designed for 'proper' outdoor use. They have a 40% off online sale at the moment.
LandsEnd has a 50% off offer for coats at the moment - their 'squall' jackets are reckoned to be pretty good.
If you are regularly going for long walks in the rain, you'll probably also need waterproof trousers (breathable ones vastly preferable) and waterproof boots.
And remember, to keep waterproof clothing waterproof, you need to wash it in the right sort of detergent: Nikwax Tech Wash or similar - several brands are available.