"Sorry mate - it's probably yer aerial - could be expensive that. Need to go and get me ladders, couple of hours work, then there's the VAT...."
To receive terrestrial digital signals satisfactorily, you need an antenna which covers more of the UHF TV spectrum than the one which receives old-fashioned analogue BBC/ITV from your local transmitter. In other words, a wide band antenna, not just a high gain one. Because the multiplex channel carrying some terrestrial digital TV services in your area will probably fall outside the antenna bandwidth needed for the 4 or 5 analogue TV channels.
Whilst I'm no fan of Sky's methods, their digital signals do at least cover the whole UK - and a Sky system can cost less than having a wide band UHF TV and terrestrial digital receiver installed......