Long Layover in LHR
 

The visa exemption only applies to same day airside transit. The moment you enter the UK you need a visa

Yes you can.

Read terms & conditions on Transit Without Visa Scheme (TWOV) on Immigration Rules Appendix V: visitor rules (www.gov.uk)

Border Force must be must be satisfied that the individual meets all the requirements and assess if an individual is a genuine TWOV

SF

not true....there are 2 types of transit visas....the first is the direct airside transit visa which is for 24 hours & you have to remain airside....the second is the visitor in transit visa which is valid for 48 hours & it permits you to change terminals, airports & even enter the uk....an indian passport holder with a valid visa & ticket to the u.s. can avail the benefits of both these transit visas without having the need to apply for one....

12 hour layover will definitely be considered reasonable....

What is the process? Just show up in immigration or should we apply in advance?

Is there any fees?

depends on where you are going & what type of visa you have....

US B1, typically connects thru LHR on the way to US.

then you don't need to apply for a transit visa....just show up at immigration & there is no fee....

Thanks. Have you or anyone done it?

https://www.gov.uk/transit-visa/visitor-in-transit-visa < This Page quotes the cost of Visitor in Transit Visa. But also the visa requirement questioner says:

my brother did this recently....his wife & him had uk visas but his daughter did not....on their way back from jfk they had an 18 hour layover at lhr & wanted to go into the city....all 3 were permitted to do so....

+1 Know many who have done that on way to USA from India. (Note that it is still dependent on what particular immigrant office considers "risk". So be extra polite and well behaved even if LHR immigration line is a zoo )

If not familiar with London, just take tube to Piccadilly and take one of those hop on hop off bus (get off only at few select stops like buckingham palace, bigben, tower bridge)

Warning: Tube back to airport can break down or get severely delayed. Plan to reach Airport 3.5 to 4 hours before flight. Security lines can get crazy too.

Given the time it will take to go through security, reach central london and back with 3-4 hours buffer before flight, you may have barely time to do full round of HOHO bus without even getting down in which case, I wud just buy the day ticket on tube (remember to collect it after putting through machine - with day ticket you dont need Oyster card) and take airport line to Hammersmith, Change to central line (which is on tracks across on same platform) and get off at Westminster. After strolling around to see Abbey, BigBen, crossing bridge for better views of Parliament, London Eye, either walk or take bus to Trafalgar Square and from there to Piccadilly and then head back. If you have more time then walk the Mall to Buckingham palace and then take tube back from nearby Green Park station.

It isn't that bad. Most issues, like signal failures, get cleared in under 30 mins. Plus Piccadilly services to Heathrow have a priority over services to other branches on the line.