As far as I know (could be wrong) although certain areas are going full digital and a retune, there not going full power until after the total switch off of analogue, sometime after the Olympics (2010 / 2011).
The Bandwidth of digital TV (470 - 900 MHz) is far too wide even for a Digital TV Yagi Aerial, let alone the old Analogue ones.
Whilst the Digital signals are low power, compared to Transmitter of Kw's of power for Analogue TV, unless you’ve got the right Antenna and coax set up its could be a non starter, even after a retune of frequencies.
Use a quality ‘Digital Antenna’ as high as possible and line to site of the best Transmitter Site. Also decent coax, CT100 (min Sat Spec) should give best results.
Also bear in mind that Digital TV Signals are not currently transmitted on the old Fill In transmitter sites (Relay Stations) which you’re old TV maybe pointing to.