Inspired by IAHtraveler's thread, now a sticky in this forum, and the many contributions of frequent Avis renters across North America, this is the beginning of a similar, perhaps more modest, thread for the UK.

Some important notes, especially for North American renters:

• Unlike in the US and Canada, the website of Avis UK continues to refer to car classes by both descriptor and letter. The full list of vehicle categories for the core fleet can be found here, Select Series can be found here, and Avis Prestige can be found here.
• Although we have letter classes, they are not comparable with those in North America: a "class E" gets you a Chevrolet Impala in the USA and a Ford Focus here.
• There is no vehicle category for any size of SUV in the UK: if you want one, you have to pony up for the Select or Prestige series and remember that we pay about $9 per gallon for regular unleaded...

We look forward to seeing you in the UK soon ^

Avis category: Small Cars

• Class A: Mini (manual transmission)
  Avis example: Peugeot 107
  Fiat 500 (officially Select Series, found at Belfast City Airport, a non-Select location)
  Kia Picanto (found at Avis franchisee at LSI, not main fleet)
  
  
• Class B: Economy (manual transmission)
  Avis example: Peugeot 207
  Ford Fiesta
  Peugeot 208 1.2L petrol
  Renault Clio
  Skoda Fabia
  Vauxhall Corsa
  VW Polo
  

Avis category: Medium Cars

• Class C: Medium Compact (manual transmission)
  Avis example: Vauxhall Meriva
  Peugeot 208 1.4L petrol
  Vauxhall Astra 1.4L petrol
  Kia Cee'd (found at Avis franchisee at LSI, not main fleet)
  Nissan Note (reported ex-fleet, October 2011)
  
  
• Class E: Medium Intermediate (manual transmission)
  Avis example: Ford Focus
  Audi A3 (also available as guaranteed make & model in Select Series)
  Nissan Note
  Peugeot 308
  Renault Mégane
  Vauxhall Astra 1.6L petrol
  VW Golf
  
  
• Class I: Medium Compact (automatic transmission)
  Avis example: Peugeot 308 Automatic
  Chevrolet Lacetti Automatic
  
  
• Class G: Medium Station Wagon/Estate (manual transmission)
  Avis example: Peugeot 308 SW
  
  

Avis category: Large Cars

• Class D: Large Compact (manual transmission)
  Avis example: Peugeot 3008
  Chevrolet Cruze
  BMW 1 Series (2011 & 2012 models, also available as guaranteed make & model in Select Series)
  MG 6 (aka SAIC Roewe 550)
  
  
• Class F: Large Standard (manual transmission)
  Avis example: Vauxhall Insignia 1.8
  Audi A4 2.0 Diesel (now Select fleet)
  BMW 3 Series (2011 & 2012 models)
  Peugeot 508
  VW Passat
  
  
• Class H: Large Station Wagon/Estate (manual transmission)
  Avis example: Volkswagen Passat 1.8 SW
  Peugeot 508SW
  Vauxhall Insignia
  
  
• Class O: Large People Carrier (manual transmission)
  Avis example Renault Grand Scénic
  Vauxhall Zafira (reported ex-fleet, March 2012)
  
  
• Class L: Large People Carrier (automatic transmission)
  Avis example: Peugeot 5008 Automatic
  
  
• Class J: Large Standard (automatic transmission)
  Avis example: VW Passat 1.8 Automatic
  Peugeot 3008 Automatic
  Peugeot 508SW Automatic
  BMW 118d Automatic
  
  
• Class K: Large Executive (automatic transmission)
  Avis example: BMW 3 Series
  Mercedes C180
  
  
• Class N: Large Luxury (automatic transmission)
  Avis example: Mercedes E200 CDi
  BMW 520d

Avis category: Select Series

All vehicles in the series are "guaranteed" make and model.

• Select Class A: Mini
  Fiat 500
  
  
• Select Class B: Economy
  Audi A1
  
  
• Select Class C: Medium Compact
  Citroen DS3
  
  
• Select Class D: Large Compact
  BMW 1 Series
  
  
• Select Class E: Medium Intermediate
  tbc
  
  
• Select Class F: Large Standard
  Audi A4
  
  
• Select Class G: Sports
  VW Scirroco
  
  
• Select Class H: Convertible
  VW Golf Cabriolet
  
  
• Select Class F: Large Standard Automatic
  BMW 1 Series
  
  
• Select Class K: Large Executive
  Audi A4 Avant
  
  
• Select Class L: Large People Carrier (Automatic)
  VW Sharan
  
  
• Select Class M: SUV (Automatic)
  Volvo XC90
  
  
• Select Class N: Large Luxury
  BMW 5 Series
  
  
• Select Class P: Low emission (exempt from the London Congestion Charge)
  VW Golf Bluemotion

Avis category: Prestige

All vehicles in the series are "guaranteed" make and model.

Aston Martin DB9 Coupe
Aston Martin DB9 Volante
Aston Martin DBS
Aston Martin V12 Vantage
Aston Martin V8 Vantage
Aston Martin V8 Vantage Cabriolet

Audi A3 Cabriolet
Audi A5 2.0 Sportback
Audi A5 2.7 Sportback
Audi A5 Cabriolet
Audi A5 Coupe
Audi A8 SE Exec 3.0 TDI quattro
Audi Q5 4x4
Audi R8 V10
Audi R8 V10 Spyder
Audi TTS Coupe
Audi TTS Roadster

Bentley GTC

BMW 3 Series Touring
BMW 330d M Sport
BMW 5 Series
BMW 5 Series Touring
BMW X3 4x4
BMW X5 4x4

Ferrari 430 F1 Coupe
Ferrari 430 Spider
Ferrari 458 Italia
Ferrari California

Jaguar XF
Jaguar XJ 5.0 V8
Jaguar XK R 5.0 Cabriolet
Jaguar XK R 5.0 Coupe

Lamborghini Gallardo LP560-4 Spyder

Maserati Quattroporte S

Mercedes C Class Estate
Mercedes C Class Sport
Mercedes E Class Cabriolet
Mercedes E Class Coupe
Mercedes E Class Estate
Mercedes E Class Sport
Mercedes ML350 CDI 4x4
Mercedes S Class
Mercedes SL500
Mercedes Viano 8 Seats

Nissan GTR

Porsche 997 Turbo PDK (Gen 2)
Porsche Cayenne 4x4
Porsche Cayenne GTS 4x4

Range Rover Discovery 4 HSE 4x4
Range Rover Sport Autobiography 4X4
Range Rover Sport Supercharged HST 4x4
Range Rover Vogue 4x4

Toyota Prius Hybrid

Volkswagen Golf 2.0 GTD
Volkswagen Golf R
Volkswagen Scirocco
Volkswagen Touareg V6 TDI 4x4