WiFi depends on too many factors to accurately say what is causing you to get less than the maximum speed.
If you’re plugged into the router and are still getting those speeds I’d be looking at what your contract states are the tolerances for advertised speed vs received speeds.
Here is some information that will help explain it to you:
Alot of routers will show what speed they are syncing at, see if yours will
Yeah as @kolok mentions theres two things to check.
You have a sync rate and throughput.
So the sync rate (in the Vodafone app) is the speed the router communicates with the equipment directly connected to the other end of the telephone line in the exchange. If this is crap then its down to the internal wires or your filter is dodgy.
The speed guarantee only applies to this, basically external factors to your house.
Throughput is the speed of data flowing through your broadband connection at any one time. This is affected by a local issue of using WiFI, how many people are using it in your home at one time, and also general internet congestion at peak times.
So you could have a stable sync rate of 70 Mbps but experience a throughput of 38 Mbps at that particular moment when running a speedtest using say speedtest.net.
na i already said it dont say in the app
Try logging into the router and having a look, alternatively call them up and ask for exact sync figures
Just to add have a look at the signal to noise ratio (SNR) figures and see if those are normal.
But it is something support should have looked at, to be honest the best way to sort these issues out is nagging support until it gets high enough up the chain.
I hope you gave more detail than you are here in your ofcom report. 
I remember seeing it in the app when I had Vodafone.
Try this instead in your browser
192.168.1.1
password vodafone