UK and Ireland:
* = Yes
X = No
! = Very Rare
* Alistair/Alastair
* Sebastian
X Auster
X Everett
X Milton
! Gabriel
* Lionel
Only Allister / Alistair / Alastair common (more N.I. and Scotland, seems rare England & Wales). Common as "son of" Mc
Sebastian seems to be people settled more recently, not common. I do know one.
Any Lionel seem to be possibly Jewish origin. English anyway.
Gabriel in print in old stories, seems to be very rare.
Never even seen Auster in print.
Only ever heard of Everett and Milton as a surnames in UK
Everett & Milton as first names would seem to me totally American.
UK isn't homogeneous at all.
Probably you could divide UK
N.I.: East & West of River Bann, also Antrim Coast very West Scottish.
Scotland: Western, Highlands, East, Lowlands/Border. I think Western Isles are very different from Shetland & Orkney (More East Ulster connection vs more Scandinavian?)
England: North, South East/Home Counties, Midlands, Marches, West Country (Cornwall is technically a Dutchy and not part of England)
Wales: North, South and Border
Isle of Man (Irish Sea) isn't actually UK. There are local Manx names and English Incomers.
Channel Is.: Parts are quite French. Also technically not UK. But not in same group as IOM.
It used to be said that only people from Asia, N.I. Caribbean etc. would much refer to themselves as British. Though some from N.I. get a bit confused when in England, others in N.I. call themselves Irish. Three counties of Ulster are not in N.I., but confusingly British N.I. people refer to Ulster as if it's N.I. N.I.'s Commercial TV station was called Ulster Television, now renamed to UTV. A subsidiary is UTV Ireland. BBC's Local Radio is called Radio Ulster.
The UK is a strange "Nation" as England, Scotland and N.I. were/are 3 separate Kingdoms (there were Acts of Union), a Principality (Wales), a Duchy (Cornwall) plus associated odd bits (Isle of Man, Channel Is., Gibraltar, Falklands plus some left over bits of Empire).