Halfords is just an auto parts store, yeah they sell headunits but their range is limited and staff are under trained in headunits.
If you want really good help.
Go to a store that just sells car audio products.
The staff will be better trained, they will have stock of primary and secondary harnesses.
I went down to a local one and got a Kenwood DNX4150DAB for my impreza.
I have an android phone, I would've loved to get an android based HU with a modern android UI that was cheap but none fit that bill.
The DNX4150 has it all for me.
GPS, USB, Preouts, FM, AM, Touch screen, BT, DVD reader.
If you're not intrested in DAB or GPS you can reduce the price quite a bit.
But overall you will be looking at 2 main kinds of 2din headunits.
The 6" type which I prefer but these you may have to have a bezel around the unit to fit around the dashboard fascia.
Then there is the 7" types
I disllike these units as the LCD portion sticks out because the LCD flips out of the way to allow you to insert CD/DVDs as well as SD cards.
This means another part to possibly break
Be it someone bumping into the LCD when it is down damaging it or the motor mechanism failing.