In all honesty the second part of your question is completely subjective. There is no single manufacturer that could possibly claim or be awarded 'the most reliable' or for that matter the best UK based support - software support is typically centred from the HQ country of origin or for hardware faults, from the company that sold it to you.
The differentiation is usually between commercial and residential products, even then many manufacturers now offer a mixture of both.
Saying that most network hardware is usually very reliable from a hardware perspective in my opinion. What is equally if not more important is how well software and firmware support is given and faults and weaknesses are patched and updated.
I see.
Are there any manufacturers that stand out in that regard?
I'd recommend Asus and Netgear kit. Not so much for the warranty and direct support aspects, but more for the fact that they make decent performing hardware and are fairly quick to patch any router vulnerabilities with updated firmware.
On smallnetbuilder.com forum section (probably the best place on the web to discuss all things about routers and internet hardware) you have quite a lot of router guru's there and there's a couple of users on there who create third party firmware for Asus (Merlin) and Netgear (Voxel) hardware.