I've seen them advertised on ebay for ridiculously low prices e.g £2.99 for 3 yrs subscription.
Can these offers be genuine?
No! Anything selling for that price can't be offering a proper VPN service, and is almost certainly designed to farm your data. Remember, you are giving them full access to all the data you send back and forth over the web, and not every site uses encryption.
Be very careful about your choice of VPN, only go for the big trusted names. Any of them advertising solely on eBay should be avoided like the plague, especially so if the eBay price is a tiny fraction of the official one! eBay and the likes are a scammers paradise with little oversight of what gets listed.
Carefully research any you're considering on the trusted tech review sites, TechRadar for example.
I assume Nord VPN can hide from the BBC the fact that it is a VPN.
The likes of iPlayer detect the use of VPNs through lots of people arriving on their site from the same IP address. Some VPNs don't have a lot of infrastructure, so may only have a small range of IP addresses they can use in the UK. A few thousand people all using this small range of IP addresses signals to the BBC that this is a VPN, and they then block requests from that address.