I don't actually feel they're lying if they don't explicitly impose a cap on the amount of data you can download.
The speed may vary due to the traffic from other users, and the capacity of the core network and peering, but they're neither saying you can only download so many gigabytes a month, nor saying you've got a 1Gbps connection but you can only download at 500Mbps.
There is a contradiction:
1. They say they never throttle your speed
2. They say they never limit your downloads/uploads
3. But they only charge £500 per month for a 10Gbps circuit
A genuine 10Gbps port in a UK data centre will cost you upwards of £2,000 per month, depending on who you buy it from.
Therefore, I think that if you really did use 10Gbps continuously, they would be knocking at your door saying "excuse me sir/madam, but unless you change your behaviour we're going to have to disconnect your service or charge you more per month".
They are probably banking on no home or SME user ever using anything like 10Gbps in practice.