I can speak from humbling experience that this issue is not always the ISP's fault.
What you need to do is disconnect every other device except the TV box, and see if you still get the same problem. You also need to connect the TV box directly to the ISP's equipment. If you don't have the same issue any more (that's what happened with me), then it's one of your other devices hogging the bandwidth. Reconnect them one by one and retest the TV each time until you find the one causing the issue. Some devices connect intermittently so this may take a while.
It does no harm to test high likelihood devices first, for example this one:
I suspect … when my son is using skype/Online gaming etc.
If it is that device, then you've found it sooner. If not, then you can stop pointing fingers and start looking for the real cause.
In my case it turned out my wife's Mac was screwing up bandwidth and causing latency issues for our gaming. We didn't confirm 100% but my assumption was this: She had set up Apple's service "pay extra to store stuff in the cloud" that syncs data between her iPhone and Mac. By "data" this means photos and videos which are large. She doesn't use her Mac all that much so for a short time there we would just disconnect her Mac when gaming. The problem eventually went away; I assume because all the major data was synced. Had the same issue a few months later when she got a new iPhone; I assume it was resyncing all the data with the phone.
If it's a device on your network, setting up QOS (Quality of Service) is the solution. Give the TV box High priority and everything else Low, and possibly your son's gaming machine Medium. If your router doesn't have QOS options, get one that does; its a pretty common feature so there should be a device in your price range that has it.
On the other hand, if you've proved its *not* another device on your network you have two more tests to do. First try a different TV provider service to confirm the issue is not isolated to that one provider (maybe its the video server that can't keep up).
Next you want to prove its not a bad TV box. Borrow a similar TV box (if necessary buy from store then return afterwards although ideally a friend or neighbor will loan you theirs, or maybe you want a second one anyway for another room) and confirm that also has the same issue.
If so -- this means you are ready to call your ISP. Disconnect everything and connect your TV box directly to the ISP's equipment, call the ISP, and explain what's happening. It will take hours and they'll start by making sure you've done all the tests above but eventually they'll accept it is their issue. In the US they would then fix the bad piece of equipment on the line; I assume in the UK it would be the same. Its possible they'll come back and say they know exactly what it is but you aren't paying for the service that will fix it. That would be frustrating but at least you can choose to pay or not pay, and also now you have another feature to look for when comparison shopping.