Is it advisable to use a rather short stopover at London Heathrow to have a quick look at some places in the c?

You won't be able to do this. It will take at least an hour to get out of the airport once you've landed and collected your luggage and passed through the immigration formalities. It will take about two hours to go through it all again + security (could be an hour because of long queues) to board your LA flight.

The Heathrow Express to Paddington takes about 15 minutes (and costs £35 return) but you would be lucky if one was just departing when you got to the platform. You could have up to a twenty minute wait if you arrive at the station at the wrong time.

Getting from Paddington to Piccadilly Circus or Westminster is about 15 minutes actual travelling time on the Tube, but you have to walk from the arrival platform at Paddington to the Underground station and then wait for the train.

You would also have to change Tube trains to get your destinations. Allow at least five minutes to walk around the Underground stations to change platforms - they're often big, complex and sprawling, not compact and neat like Berlin UBahn.

Then you would have to do it all again on the way back.

Remember that Heathrow is a large and complex airport so just getting around the airport during your transfers will take a lot of time.

BTW you won't see Big Ben. That's the big bell inside the clock tower. Just standing outside the Palace of Westminster (Houses of Parliament) and looking at the tower will leave you wondering "was this worth the cost and effort?" You'll feel the same if you only have five minutes in Piccadilly Circus - it's just a famous roundabout. You should plan a long weekend in London and then you can take your husband around properly. Include the London Eye, the Monument, boat trip on the River Thames, Hyde Park etc.

4 hours is not long enough. You need to allow for delays, checking in and out and in again. Heathrow to Paddington may take 15 mins in theory but you then have to get to Westminster, which would take another 30 mins. There is a 2 hour check in before European flights.

Far too short a time, as can take an hour to get into Central London and you have to get through immigration.......Would not entertain the idea.......You would be boarding your 12 o'clock flight around 11.30, at the very latest, and an 8am arrival could take a while to get to its gate etc......

You don't really have enough time to go anywhere. You would be better off booking seats in an airport lounge so that you can chill out and relax.

Glorious.

No, personally I don't think you will have the time................... everything ontime...8 am landing, through customs at best 1 hour = 9am, you need 2 hours prior to your flights means 10am....so you have 1 hour to get there and back and see what you wish to see......................................