Very much depends on the walk. Some I plan for weeks, usually because it is a first walk in a particular area. For those I use Bing maps Google Earth, Walking Highlands. Then for walks on Dartmoor there are firing times to check and from start to finish of any plan there is the weather. For that there is only one really good site for the UK, the met office.
I have A4 laminated sheets covering Dartmoor and Bodmin moor that I carry with me. To be honest I seldom look at them. If the weather comes down my phone and OSmaps app are far less fuss. Though not infallible. Even in new areas I have the good fortune to normally be able to memorize a decent map a week or so ahead of a walk. If it is something you enjoy and I suspect many here are the same, then pouring over maps for hours memorising details is a real pleasure.
For a day in the Brecon Beacons, a 4 hour drive each way, there is the car to check over fuel, tyres, washer, oil, snow chains for winter. Food to prep the night before and the usual kit list to chuck in the day sack.
One thing I have never yet tried is following a GPS plot. Don't quite get why so many people do.