When you self-transfer (buy separate tickets, which is the only option with most low-cost carriers), many airlines will consider only their own flight and nothing else. They don’t care about transit, connections or onward flights: for them they are transporting you from A to B, and you need to have documentation to enter B as a regular visitor, even if you are actually just transiting to C.
If this is the rule they applied, then it is perfectly normal for them to deny boarding: to enter the UK as a visitor, if you are not a citizen of a visa-free country, you would need a visa, with a limited number of exceptions.
They do this for multiple reasons, including:
- It’s easier and quicker for them if agents only have to check against a single set of rules (which is already complex). If they accept transit, they have to check that you are eligible to travel to your final destination and to fly to the intermediate destination for transit. Transit rules are often extremely complex, as they often depend not only on your citizenship, documents and destination country (like for entry), but also on the actual airport, the combination of airlines, whether you have checked bags or not, whether you will need to go through passport control or not, and so on.
- They would have to check that you actually have an onward flight meeting the criteria. They have no way of doing so if it’s a different airline, and it’s easier if they are consistent.
- Since the two flights are booked separately, if you miss your second
flight, you may be stranded at the connecting airport, and then they would have an issue with the immigration authorities of that country (having to fly you back and having a fine to pay).
Things would be different if you had actual connecting flights, booked on a single ticket: in that case the airline would have taken into account your whole itinerary, checked for documentation for transit in connecting countries and entry in the destination country. Also if there are any issues with the connection they have to take care of you and rebook you to your final destination (or fly you back if this is your choice). All of this costs them more, which is why LCCs don’t want to do it and only sell point-to-point rather than connecting flights.
So you have two options:
- Either you book actually connecting flights, and then documentation requirements for transit at intermediate airports are actually valid;
- Or you check requirements for entry (as a visitor, e.g. a tourist) for intermediate countries where you self-transfer (this would require a visa for the UK).
Of course the simplest option is a direct flight if the option exists (it does in your case).
You could argue that the UK rules do not mention anything about a self-transfer or actual connecting flights and they the denied boarding was unjustified. I have no idea which way this would go in court and if you thus have any chance of getting a refund and/or compensation. I’m not aware of case law in either direction. You may want to enrol the services of one of the specialists who claim for you (in exchange for a hefty commission of course), they would probably be able to tell you quickly if you have any chance of success or not.