I didn't think about the fitting scenario. But, I agree with you, I wouldn't ask Halfords to fit anything, if for no other reason that I know how to do regular service replacement things. I have seen too many stories of Halfords staff making a dogs' danglies of simple things, like changing wipers or bulbs and getting it totally wrong !! Duuuuh !
I checked under the bonnet of my vehicle, this morning, and there does seem to be a bit of leeway with the positive battery lead, so that it is possible to fit a battery with a centre terminal configuration, such as the Hankook MF31-1000. My car is a 2001 and looking at the positive terminal clamp there are , I think, three cables connected in the clamp. There are two heavy, red, ones, one going to the alternator, I suppose, one going to the starter motor, and one going to the fuse board. This is only from memory, and they are factory fit and not modified. There is a bit of movement on the clamp which would allow an inch or so of sideways movement.
The difference between a side edge terminal post configuration and a centre post configuration is probably only an extra inch or so.
One other annoying thing is that I emailed three different battery suppliers, two are distributors and the third was Yuasa UK, and I haven't heard back from any of them. There are two versions of the Yuasa HSB334 battery depending on where you look. Halfords list the battery as "HSB334" but other suppliers list an identical one as "Yuasa YBX5334". So, I emailed two suppliers, and Yuasa UK, but no-one has replied to me to explain the difference. The only difference that I can see is that on the Halfords item there is a label that says the battery has a 5 year guarantee. The other battery, although exactly the same specifications, doesn't. Maybe this is the difference.