i have a flight from Hong Kong to Berlin via London with Cathay pacific
I would like to do whether it is possible to go to town before my flight? It's about 9hrs in transit...do I need to pay? As it doesn't allow stopover.

Thanks

Do you have a passport and/or visa that allows entry into UK?

Now if you do not tell the airline you are going land side?? @:-)
Do not volunteer information to airline staff or immigration/custom unless asked. (But tell the truth)
Many people take the tube/train into the city during long transits

Assuming you are travelling to Berlin with BA from terminal 5 just make sure you are through security at least 40 mins before departure or you will not be allowed to travel.

I don't think that there is any regulation that force you to remain airside during a stopover, as long as you have the proper documents for entry in the UK.
Just make sure that you allow ample time to clear immigration and security when you return to LHR.
BTW, I believe that conformance time for BA is 35 minutes. And that is when your BP is checked in the security queue, somewhat before getting to the security machine, not after having cleared security. AB seems a bit more relaxed on conformance.

I have done this many times with transit times of 6 to 7 hours on a through ticket which does not explicitly break my journey in London. As long as your passport is OK, and you time yourself sensibly you'll be fine.

Same for me. I often book connections with just less than 24 hours to break a long journey up. Never a problem, nobody ever asked.

I have done it a couple of times without a problem. When asked at immigration how long are you going to stay in London I answer just for lunch. Typically a chuckle and have a good lunch and I am on my way.

Agreed. I routinely grab the Express to Paddington just to get a coffee with a 4-hour connection. The fresh air is great after a long haul arrival in LHR.

Just write "transit" on the customs cards. If they ask just tell the truth - x hour layover and you are going into town for a couple hours.

9 hours is plenty of time to get in and out of the city.

The quickest way (and most expensive) is the Heathrow Express to Paddington. Give yourself 30 mins for the journey (15 on the train plus time to walk through endless passages to the train and off the train).

Alternatives are heathrow connect which is cheaper and slower (not recommended for your short stopover) or the Picadilly tube line. This is good if you want to go somewhere on the Pic line as it's probably just as quick as the HEX after to take into account the changing of tube lines and a fraction of the price.

On the Picadilly line you can go to South Kensington (for museums), Knightsbridge (for Harrods), green park (for Buckingham palace), Leicester Square, Picadilly or Covent Garden.

Heathrow to green park is about 45 mins

Give yourself plenty of time to get back into heathrow though just in case there are delays

Enjoy

Jon

Sure you can. I did it many times as long as your passport allows to do so.

9 hours is plenty of time. I would get the Heathrow Connect (I refuse to be extorted by the HEX) to Paddington then jump on the Bakerloo Line to Piccadilly Circus, or even Waterloo and walk up to Westminster/Buckingham Palace/Bond Street from there.

A nice alternative if you've done London before would be to get a taxi and spend a few hours in Windsor.