With your situation I believe getting a pre-built gaming PC would be the best way to go. They are already assembled, all parts compatible, and ready to use right out of the box. The main disadvantages are that they can be more expensive than building it yourself since you are also paying for professional installation.
I’m not from the UK, but googling “UK custom PC builders” brought up a lot of results.
Looking at one retailer, Overclockers UK, they have some decent options for less than £1500. That is only one retailer, you’d be best to do more research and look at other retailers.
As for viruses, I’d recommend getting a standalone anti-virus software. Typically Window’s built in security features are good enough as long as you use AdBlock on your browser and have common internet sense, but kids being kids are going to find ways to bypass it or do something they shouldn’t have, like looking for free download links for games you need to pay for. Getting a standalone virus software can be more secure and may have features such as parental setting to help make sure your children are safe on the internet also to protect the PC from viruses, malware, etc. I don’t use separate antivirus, but if I ever have kids it is something I’d absolutely get.
According to a few websites I read often, Bitdefender seems to have the best anti-virus. You can get it for free or pay an annual fee to get the “Pro” version. Would be worth it if the PC comes out under your budget.
If you find some PC’s you’re considering buying and want to share them here for our opinions, we’ll let you know if they will be good for your kids’ gaming