Unfortunately nowadays most of us transmit 'blind' not having the separate TX/RX units of yore or any other old-fashioned test equipment to monitor our transmissions. So the first thing to do is actually confirm to yourself your transmission quality rather than rely on someone else's word.
We do have online receivers at our disposal that we can check with if they are close enough to pick up your transmissions.
Take a look at the KiwiSDR map on the following link :-
You'll have to zoom into the UK on the map and the nearest receiver to you is the Wessex one by Martin - G8JNJ. I can't link directly unfortunately.
With this you should be able to experience for yourself your own radio transmission quality.
There are obvious ways of 'ruining' your transmissions, power microphones, mic gain turned up too high, shouting into the microphone or talking too close to it all can cause 'clipping' or audio distortion. These are easily rectified.
The other is RF coming back into your radio system or 'common mode currents' and you should be able to identify this by seeing if your SWR varies whilst touching the chassis of your radio on transmit. Don't do this on high power or else you could get an RF 'bite'! If this is the case then common mode chokes usually alleviate it including the simple coax wrapped around a pipe solution to make an 'ugly' or air wound choke.
The above are 'generalisations' as you haven't given any specific information to your woes such as transmission mode (SSB/AM/FM) or frequencies in use.
The only other suggestion I can give is for you to get independent audio reports from as many different stations as you can to actually verify your audio quality. Sometimes the problem can be at the receiving end rather than yours.
I hope that's of some help and wish you well with it all.
All the best,