I get asked this a fair bit so I thought I'd put it all in one place. Most of what I buy comes through a handful of channels, some online, some more local. Here's what's actually worked for me.
eBay is still the single best source for secondhand commercial kitchen kit in the UK, and I don't see that changing any time soon. The trick is to set up saved searches for the specific brands and items you're after and just check them regularly. I've got saved searches for Rational, Winterhalter, Classeq and a few others, filtered to used and "for parts or not working" when I'm looking for stuff to fix up. Collection-only listings are where the real bargains tend to be because most people can't be bothered driving to pick something up, so the prices stay low. If you've got a van and you're willing to drive an hour or two, you're already ahead of most of the competition.
For auctions, John Pye in Preston is worth keeping an eye on. They do a lot of commercial and industrial stuff, including catering equipment from closures and insolvency jobs. You can bid online but it's worth viewing in person if you can because the photos don't always tell the full story. Pro Auction run restaurant and hotel closure sales across the country and they post upcoming ones on their site, so it's worth checking what's coming up in this area. The other one is i-bidder, which is more of an aggregator. Most of the regional auction houses list through there. You can set up alerts for catering equipment in Lancashire and you'll get an email when something comes up. I've had some decent finds through there.
Facebook Marketplace is hit and miss, to be honest. Occasionally someone lists a piece of commercial kit who doesn't really know what it's worth and you can pick it up for not much, but it's not reliable enough to depend on. Worth a scroll every now and then but I wouldn't build a strategy around it. Same with the local Facebook selling groups.
The other thing that's worked for me is getting to know a couple of the clearance companies that handle restaurant closures in the area. When a place shuts, they're usually the ones dealing with the contents, and sometimes you can buy bits directly before it goes to auction. I'm not going to name the specific ones I use because I'd rather not create my own competition, but they're out there if you look. It's worth building those relationships because you hear about stuff before it's listed publicly.
Lancaster's not massive but there's enough hospitality trade that things come up fairly regularly. Morecambe too. And being within reach of Preston, Blackpool and the Lakes means the catchment area is decent. Lots of hotels and restaurants turning over in the tourist areas, which means kit becomes available fairly often.