You need to pay for rack space, you need to pay for power/cooling, and you need to pay for transit.
Colocation in the UK tends to be pretty expensive. Unless you're able to get it for free / at a significant discount on mates rates, it's generally unviable.
If you've reach the point that it is economically viable for you (considering the up front cost of the hardware + ongoing colo costs), you now need to factor in the ongoing support costs of the hardware. You may get lucky and have nothing break (happened to me twice over ~8 years), you may get unlucky and lose PSUs, DIMMs, disks, fans, controllers, etc. The severity of this depends on your particular hardware configuration. When things break, you either need spares on site, or you need to take/ship spares to site. You now need to either get remote hands to fix this for you, or you need to fix it yourself.
Honestly, you're almost always better off just using one of the cheaper cloud providers and/or renting bare metal than trying to do this.
However, if you've determined that you're in that sweet spot where it does actually make more sense to colo a server (as I did for many years), then go for it, and have fun!
Like a lot of other commenters have noted, make sure you have an IPMI/ILO/DRAC interface to your server, you probably want that sitting behind a firewall rather than being directly exposed to the internet (so now you need a real firewall as well to protect your IPMI interface) unless you want your server pwned in no time.
If you're thinking you can't do it at home due to space / noise constraints, look at smaller sized units like NUCs, laptops, or other SFF PCs that can be used as virtualisation devices. You wont get as much bang for your buck as you would from a used rackmount server, but the administrative overhead costs are a lot lower than dealing with DCs.