There is no real easy answer to this question. What you can do however is walk out into your street ( if you have neighbours!) and have a look at the television aerials that your neighbours have had fitted. This is often a huge clue as to what type of aerial installation and type of television aerial you need. If your neighbours have roughly a 6ft mast, with a basic looking aerial on the top and your aerial installation engineer is recommending a 20ft mast with a high gain multibeam aerial on the top, ask them why! There may well be a valid reason for installing an aerial on a 20ft pole, even if your neighbours have small aerials. This should however be very obvious and definitely the engineer should be able to explain and justify the type of aerial and installation they are recommending. Ask questions. Never feel pressured.

Recommended aerial installation companies or installers

You cannot beat a recommendation for an aerial installation. If you are lucky enough to source an aerial company that has already got a proven track record, from somebody or a company that you know, that is your 1st point of contact. Thereafter probably the best thing to do is to look up the CAI and look for the: Find An Aerial Installer page.
Here you will have listed lots of companies that are highly likely to be reputable.

Aerial Installations by TelevisionAerial.co.uk