Cabin Baggage Allowances & Luggage Sizes
With the charging structure of some "low cost" carriers, the cost of checking baggage can sometimes be as much as the ticket. All airlines do allow you to carry hand/cabin baggage - but the rules vary as much as the size of case you're allowed.
So here is our guide to the major UK carriers, and some comparisons.
In all cases hand bags, briefcases, laptops and laptop bags all count as one piece.
Aer Lingus - Hand Luggage Allowance

1 piece of luggage. Dimensions: 55cm x 40cm x 20cm (22x16x8 inches). Weight less than 10 kgs (22 lbs) and be small enough to fit in the overhead locker or under your seat.
Excludes Aer Arann flights whose dimensions are: 48cm x 36cm x 21cm (19x14x8 inches). Weight 7kgs (15lbs).
Air Southwest - Hand Luggage Allowances

1 laptop case plus
1 piece of luggage, maximum dimensions: 56cm x 45cm x 25cm
Weight allowance: 10 kg.
If flying from Leeds, Cork or Dublin the laptop case must be within the single item of hand baggage.
BMI Cabin Baggage Allowances

All Economy and Flexible Economy ticket holders with BMI have an allowance of 1 piece of hand baggage.
The maximum dimensions including wheels and external pockets must be no more than 55x40x23 cm. In addition they are allowed 1 small personal item eg. handbag, small briefcase or laptop.
Those flying Business Class have an entitlement of 2 bags plus a personal item, subject to the same dimensions.
There are currently different restrictions and allowances for those flying to Damascus, Dammam, Jeddah and Riyadh so please check with the airline before travelling.
For additional information:
http://www.flybmi.com/
BMI Baby - Hand Luggage Allowances

Travellers with BMI Baby are permitted 1 piece of hand luggage per person, with maximum dimensions 55x40x20cm and a maximum weight of 10kg.
Dimensions are to include the wheels and handles of the bag.
Official website for more information:
http://www.bmibaby.com
British Airways - Hand & Cabin Luggage Allowances

2 pieces of hand baggage are allowed.
The first can be up to 56cm x 45cm x 25cm - including the handle, pockets and wheels.
The second (laptop or handbag) must not exceed 45cm x 36cm x 20cm.
Your laptop or handbag must fit under the seat in front of you.
easyJet - Hand Luggage Sizes & Allowances

easyJet has an allowance of 1 piece of hand baggage.
Maximum dimensions, including wheels and handles, are 56 x 45 x 25cm. easyJet do not set a maximum weight allowance, but say it has to be "reasonable". Size will change on 2nd July 2013 to 50 x40 x 20cm
www.easyjet.com
Flybe - Hand Luggage Sizes & Allowances

Flybe passengers travelling on an Economy, New Economy and Plus tickets are allowed 1 piece of hand/cabin baggage per customer.
New maximum dimensions from July 2012 are 55 x 40 x 23 cm and these must include include the bag's handles and wheels. Weight must not exceed 10 Kilos.
Keep updated:
http://www.flybe.com/flightInfo/baggage.htm
Monarch Airlines - Hand Luggage Allowance

Effective 1st November 2012. Monarch now have an either or option for cabin baggage.
Either one cabin bag of 56x40x25cm - Max weight of 10 kilos, plus a small duty free bag or two pieces of hand baggage, with a combined size of 56x40x25cm, and again a maximum weight of 10 kilos, plus a duty free bag.
Ryanair - Hand Luggage Sizes & Allowances

Individual baggage allowances for travellers flying with Ryanair are strictly one item of cabin baggage per passenger.
Maximum dimensions are 55cm x 40cm x 20cm with a maximum weight of 10 kg and this must include accessories such as handbags, a laptop, and duty free items.
Oversized hand baggage will be placed in the aircraft hold at an extra charge. Infants do not qualify for a cabin baggage allowance (although a pram/buggy will be carried free).
Keep up to date:
http://www.ryanair.com/en/questions/baggage
Thomson Fly - Hand & Cabin Luggage Allowances

Passengers flying Thomson airlines are permitted 1 piece of cabin/hand luggage.
Maximum dimensions: 55cm x 40cm x 20cm. Maximum weight 5 kg.
Virgin Atlantic - Hand Luggage Allowances

Virgin Atlantic Economy and Premium Economy passengers may take 1 piece of hand baggage on board. Individual dimensions must not exceed 23 x 36 x 56cm. Maximum weight is 13lb/6kg.
For those travelling in their Upper Class service, 2 bags are permitted. The maximum size of each bag may not exceed 23x36x56 cm and 12 kg in weight. The combined weight of both bags has a limit of 16 kg. One bag may be substituted for a folding garment bag, of 20 cm when folded and 12 kg.
For those travelling with children: They are allowed 1 bag, 23x36x56 cm and 6 kg in weight.
Number of items
For additional information:
http://www.virgin-atlantic.com
Please remember to check with your airline before you travel as rules, allowances and regulations change, sometimes with little or no notice.