Hi @PHILIPK1980, thanks for your post and welcome to Virgin Media! So happy that you've joined us.
As @Adduxi has mentioned, the speed guarantee is only valid when you're connected with a wired ethernet connection. The reason for this, builds onto @carl_pearce's point as wifi speeds are influenced by so many external factors, like device used and its capabilities, distance from the Hub and amount of devices online to name a few.
If you wish to know more about the speed guarantee, it might be worth checking this page out.
Just to reassure you too, I have had a look at your account and there aren't any faults on your network thankfully!
I do hope this information helps, and please let me know if you need any further support.