The frequency reallocations have come about as part of a Europe-wide agreement (because radio waves don't stop at national borders). They were needed to make room for the vast number of new services which constantly have to be squeezed into the radio frequency spectrum. They're part of the same agreement which will see FM radio broadcasting come to an end in a few years time.
A TV aerial should, ideally, have elements which are exactly the right length to 'resonate' with the frequency which is to be received. Since TV channels are transmitted in 'bundles', called 'multiplexes', with each multiplex using a different frequency, you should theoretically have a different aerial on your roof for each multiplex. (So you'd need about 10 aerials on your roof to receive all of the multiplexes for the main channels). That's clearly not possible, so any aerial will only be a 'rough match' to the incoming frequencies, so it will be better at receiving some multiplexes than others.
Matters are in fact even worse than that because, unless you've had a new aerial fitted since the digital changeover, the aerial on your roof was never designed to resonate with
any of the current frequencies (because analogue TV was transmitted in a completely different frequency range). So what most homes have currently got on their roofs are aerials that are little better at doing the job properly than the wire coat hangers that people used to use for car radio aerials. (The digital changeover was based around the assumption that approximately one third of households would need to have new aerials fitted. It turned out that far fewer homes needed new aerials but it did then leave lots of homes in the 'wire coat hanger' situation, so it's hardly surprising when some of them can't get reliable Freeview reception).
Your aerial clearly doesn't resonate well with the frequencies now used by the 'COM7' and 'COM8' multiplexes. Buy a signal booster from Argos:
http://www.argos.co.uk/product/5713543
(You'll also need a fly lead. They're widely available in discount stores, etc or you can get one cheaply from Screwfix:
https://www.screwfix.com/p/fly-lead-coaxial-plug-to-plug-1m/41720 ). Install the booster and carry out a further retune. (NB: If you're given the choice between 'adding' channels or 'replacing' them, ensure that you use the 'replace' one; you need to force your TV to start the tuning process from new).