Craghoppers Kiwi Pro are my favourite walking trousers. I climb mountains for fun, and need walking trousers for work, I buy extra pair of these in preference to the uniform ones we get from work. They're usually available on Amazon half price in at least some colours.
For a waterproof hillwalking jacket, you can spend anything from about £100-£500. Waterproof jackets are always only as breathable was what you wear underneath though, so you will get condensation inside the most expensive Goretex jacket in the world if you wear a big woolly jumper underneath.
I wouldn't recommend warm and waterproof in a single jacket - you need layers; base layer (long sleeved thermal vest), fleece layer (possibly 2) and waterproof shell.
Alpikit and Decathlon are very good for the price, but Alpkit was sold out of a lot earlier this week. I've never had a Mountain Warehouse waterproof, but I have been impressed with the other stuff of theirs that I have had. Land's End are worth a look too.
Other "big names" are Rab, North Face, Marmot, Montane, Berghaus (their are many others, although I hated the Jack Wolfskin jacket I had) - the Rolls Royce brands tend to be the US mountaineering brands - Patagonia, Mountain Hardware, Arcteryx. Rohan make some decent kit too.
Look out for breathable fabric (Goretex is a particular type of breathable waterproof fabric, but many manufacturers also use their own fabric) taped seams, and a label that gives the "hydrostatic head" value of at least 10,000mm. This means that the fabric will withstand a 10 m column of water without leaking, This is what defines waterproof (lower values are classed as water resistant).
You may want to try on a few jackets to find one that fits well. Jackets sold as mountaineering jackets tend to be very narrow fitting on the hips (so they don't get in the way of a harness) - which doesn't work for me!