Top end "luxury" hire cars - what do they issue?

[SillyALFALove]

Looking to hire a car for just over a week in the UK.
I need something bargetastic but am intrigued what each company classes as Luxury, Premium or whatever designation they suggest.
They often seem to be a Merc E Class "or similar" Im hoping or similar does not mean a large Kia or such like..

[farrendahl]

Merc E class or it's Audi/Lexus/BMW/Volvo counterparts are the usual bet, although the BMW is a rarity. I wouldn't be surprised if some providers also start adding Infiniti's to their fleets as well.