How to email UK border control to know whether I need a transit visa or not [duplicate]

Since the dissolution of the Border Force NICE unit, there's no Border Force unit that liaises with the general public.

Per TIMATIC, the database used by airlines:

Visa required, except for [...] making a landside transit with a confirmed onward ticket for a flight to a third country that departs before 23:59 the next day. They must [...] have an immigrant visa issued by the USA with a US entry stamp

and

Visa required, except for [...] making a landside transit with a confirmed onward ticket for a flight to a third country that departs before 23:59 the next day. They must have a standalone immigration visa form 155A/155B issued by the USA together with a sealed brown envelope

In other words: if this is when you're moving to the US, present the sealed envelope at airline check-in and at the UK border. Otherwise, just show the immigrant visa in the passport (with the entry stamp next to it).

The check-in staff at the airport should go by TIMATIC, so refer them to it if necessary. They MUST put "USA" in the "Alien residency" field, or the exemption will not show.

The UK rules, as show on the official UK check if you need a visa website state that you can transit without a visa with a landside transit (going through immigration) on the next day if:

• you arrive and depart by air
• have a confirmed onward flight that leaves on the day you arrive or before midnight on the day after you arrive
• have the right documents for your destination (eg a visa for that country)

One of the following must also apply:

• you’re travelling to (or on part of a reasonable journey to) Australia, Canada, New Zealand or the USA and have a valid visa for that country
• (...)
• you have a valid USA permanent residence card issued by the USA on or after 21 April 1998
• you have a valid USA I-551 Temporary Immigrant visa issued by the USA (a wet-ink stamp version will not be accepted)
• (...)
• you have a valid standalone US Immigration Form 155A/155B issued by the USA (attached to a sealed brown envelope)

So if you had a US PR card, you can TVOW landside the next day to/from any destination. Without a PR card, you can do so as long as you hold a valid US visa AND travel to/from the US.

This is reflected in Timatic, which states that if you are a US PR, you can TWOV if you:

• have an immigrant visa issued by the USA with:
• a US entry stamp, and
• the following statement printed above the machine readable zone of the visa: "Upon endorsement serves as temporary I-551 evidencing Permanent Residence for 1 year" (...)

Or if you:

• have a standalone immigration visa form 155A/155B issued by the USA together with a sealed brown envelope (...)

Or if you:

• have a Permanent Resident/Resident Alien Card (Form I-551) issued by the USA on or after 21 April 1998 (...)

If you are not yet a US PR (which you are not), then Timatic states that you must:

• have a visa issued by Australia, Canada, New Zealand or USA, and
• be traveling to the country that issued the visa, and

You may encounter a stubborn agent in Beirut that will consider you to already be a PR and only applies the rules for PRs, but the likelihood of this is IMHO quite low. There is no reason for UK authorities to refuse the transit (if of course all the usual conditions are met).

As you already have your ticket and it's non-refundable, I really think your best option is to go to the airport and board your flights. In the worst case, you will be denied boarding in Beirut, and that's it.