Given a travel related situation, and given a specific question about the situation, summarize the question into a title
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Question: When paying by card flight attendants usually ask to see your ID, to verify that the card in fact belongs to you. However it seems weird to me that pretty much no other merchants ever ask to see your ID, even when conducting purchases for thousands of euros (e.g. at IKEA).

So why do airlines insist on verifying your ID when paying by card? I understand that there's (usually) no Internet to verify the card balance on the plane (as suggested by TonyK), but how would one's ID help them out in case the card is empty?


Answer: Why do flight attendants ask to see your ID when paying by card on the airplane?


Question: Last time I visited the UK, 
it took 3 hours to get through the immigration queues.

I recall seeing signs saying something like:


  "Registered Travellers from [list of countries] can use the EU/UK lines."


I have another trip the the UK in a few months,
and I am really dreading the giant queue, especially after a 17 hour flight.

So I am looking up how to become a Registered Traveller.

It says:



Apply online for Registered Traveller membership. ...
You'll get a decision on your application within 10 working days. ...
If your application is accepted, the next time you travel to the UK you must go through the 'other passports' lane and fill in a landing card.
The immigration officer will check that you meet the criteria and tell you if you can become a member. ...



So does this mean that being accepted at point 2, isn't a real acceptance.
You get the real acceptance the first time you enter after applying, using the 'other passports' lane (i.e. the non-UK/EU line).
And then after that if you visit the UK again (assuming all successful), you get to use the EU/UK lane?

(I mostly just want to know if I am understanding it right. I think other things mean it just isn't financially worth it to me to become a registered traveller. I'ld need to pay for a Visa since I've not visited 4 times before)


Answer: Is it possible to use UK/EU lanes as a Registered Traveller the first time you enter the UK after applying?


Question: As the UK is technically still a member of the EU, I just wanted to be sure of this before booking travel arrangements. 

The idea is to arrive in the UK and stay there for 2-4 weeks, and then afterwards relocate to and stay in mainland Europe for another 80-90 days. Is this possible? Or would I be effectively limited to 70 days in the EU after staying for example 20 days in the UK?


Answer:
If I arrive in the UK, and then head to mainland Europe, does my Schengen visa 90 day limit start when I arrived in the UK, or mainland Europe?