Thanks to 7nen , we have an official document telling us what we can and cant do.

British Airways welcomed the Government’s announcement of Monday August 14, easing restrictions on hand baggage on flights departing all UK airports.

From Tuesday August 15 all UK airports implemented the new hand baggage policy.

Customers departing from the UK (including customers transferring through the UK):
Customers travelling from the UK will be able to take on board as hand baggage one cabin bag no bigger than 45cm x 35cm x 16cm, the size of a small laptop bag.

Cabin baggage MUST NOT contain:

• Any cosmetics
• Any toiletries
• Any liquids
• Any drinks
• Cigarette lighters

Cabin baggage CAN contain the following:

• Electronic equipment, including laptops, mobile phones and portable music and DVD players
• Essential prescribed medicines in liquid form provided they are under 50ml. Customers will be asked to taste the liquid. If they cannot taste the liquid for any reason they will be asked to go to an airport pharmacy to have the medicine verified.
• Baby milk and liquid baby food (the contents of each bottle MUST be tasted by the parent)

Nothing must be carried in pockets. For more detail of the Baggage restrictions click here

Please ensure that you do not check into the hold any essential items such as:

· Household and car keys (including electric key fobs)

· Travel documents including passport and itinerary and any important contact details

· Essential medicines for the journey

· Wallets or pocket size purses

To help progress through the airport all customers are encouraged not to include items capable of containing liquids (e.g. bottles, flasks, cans etc.) in either their hand or checked baggage.

All electronic equipment will need to be removed from the item of hand baggage and screened separately. We recommend these items be packed carefully for easy removal at the security search point.

Pushchairs and walking aids will be permitted but must be x-ray screened.

Customers may purchase any item in any store in the departure lounge before departing their final UK airport and take onto the aircraft as normal, unless they are travelling to the USA.

Extra restrictions are in place for customers travelling to the USA from the UK. Customers WILL NOT be permitted to take any liquid or gel items purchased in the departures lounge into the aircraft cabin. All food or beverage items must be consumed before boarding.

Customers transferring through the UK
Customers transferring through a UK airport onto another flight must not buy any of the prohibited items listed above in duty free, prior to transferring onto another flight.

Customers transferring through the UK will be subject to the same hand baggage size restrictions as those originating their travel in the UK.

If you arrive into the UK to connect with a British Airways flight (irrespective of original carrier), with more than one laptop size bag (45cm x 35cm x 16cm), you may be refused travel.

Customers travelling from the rest of the world to the UK
Customers departing on flights to the UK from the rest of the world who are not transferring onto another flight, will be able to carry one standard sized bag, 56cm x 45cm x 25cm and one briefcase, laptop computer bag or equivalent, unless more stringent local regulations are in force.
However, customers returning from the UK must comply with the UK Department for Transport requirements which are:-

• One cabin bag no bigger than 45cm x 35cm x 16cm, the size of a small laptop bag.

We would therefore strongly advise customers to check that the laptop computer bag does not exceed these dimensions.

Link: Answer