My Tomtom also does full postcodes....
I'm not aware of any of the satnavs that allow entry of OSGR's as they are aimed at road users, they use postcodes or lat/longs.
MY personal preference is for TomTom (even though I was a garmin GPS person).
I try to keep an open mind.....when I bought Anita a satnav I went to halfords and had a play with garmin/tomtom the others.
..............I left with a Tomtom
You might want to think about what phone you have ....I seldom us my Tom tom as I have a nokia phone and get nokiamaps for free. This allows me to run viewranger which does do OSGR's (and is also free to us) Plus it makes calls and gets me on the internet etc etc...
If you want to borrow my tom tom then you are welcome to have a play for a couple of weeks.
Mark