Well then...
that depends on how you are using the term. It can be used as an encompassing term, following a logic similar to Geordies are northerners, northerners are English, the English are British etc. cf A European car, a British company, a North American etc...

An indeed the term is sometimes used in this sense, not to refer to a specific accent but to refer to a group of accents.
( Sextant goes to him is emotionally erect, on this day, Sat 20 Mar 2010, 14:27, 2 replies)

But rather a lot of people use
when they mean to say a southern English accent.

I guess though it's similar to saying an American accent when they too differ across the country.
( Mrs Entity is in hiding., Sat 20 Mar 2010, 14:34, closed)

Yes
To Americans we probably all sound the same, just like most Amercans sound the same to me. Mind, I get confused enough between Irish and Scottish myself.
(Cloudane, Sat 20 Mar 2010, 18:59, closed)

no it can't be used as an encompassing term
There is just no such thing as a British accent. The difference between, say, a Californian and Bostonian accent and between Essex and Glaswegian is incomparable. It drives me up the wall, every time.
(jennymnemonic, Sun 21 Mar 2010, 12:14, closed)

and...
Brummie is incomparable to Geordie, yet using English as an encompassing term is somehow valid?

Of course it's an encompassing term, English, Scottish and Welsh are all subsets of it.

Just like Coal mining and Whisky are/were both subsets of British industry.
( Slippery Mick ‏, Sun 21 Mar 2010, 23:37, closed)