>>>Would a loft aerial be good enough
That's impossible to answer without knowing how far she is from the transmitter. Terrestrial TV transmission has always been upon the basis that viewers will get a good signal by using a good-quality
external aerial. i.e. there has never been any intention to provide a decent signal to people who only use internal aerials. Of course, many people do get a good signal using such aerials, simply because they're close to the transmitter (with a clear line of sight to it).
Go here:
http://www.digitaluk.co.uk/postcodechecker/main/index/dummy/NA/yes
(Make sure that there's a tick in 'Detailed view')
Enter your mother's postcode.
On the results page, look for a transmitter where all of the coloured rectangles are green. If there is one, then you stand at least some sort of chance of getting a signal via an indoor aerial. (That chance will probably be much improved if the distance to that transmitter, shown at the top of the page, is under 20km).
I'm also confuded by what sort of aerial you're considering. There not really such a thing as a 'loft aerial'. (It's just an external aerial used in a loft). You say that you want to use it "in my mum's bedroom". Does that mean that the aerial will actually be there or just the TV set?
If there's actually a large enough loft space you can use, this is the best you can get:
http://www.maplin.co.uk/91-element-high-gain-digital-tv-aerial-221124
If your mother's flat is very close to a transmitter, this might work in her bedroom:
http://www.maplin.co.uk/one-for-all-hd-terrestrial-amplified-aerial-383744
Chris