LIFO may be part of your CoS. In my company, seniority is used as the criteria for promotion, demotion and redundancy as it's written into our contract - therefore it's legally binding. However, it's not a UK comapny.
I was in Virgin during 9/11 and two fleets (747 Classic and A320) were scrapped overnight. The policy was going to be redundancy in fleet as there was no redundancy policy laid down. This was far from popular, however all the recent joiners were on the 744 and LIFO would have left the now very busy 744 vastly undermanned. After lengthy discussion with the union, a LIFO policy was agreed, but not after a lot of people had been made redundant. In the end, the 744 pilots were offered unpaid leave but none needed to take it. And no 744 pilots were made redundant - the only ones who left (self included) took voluntary redundancy.
At the time, Virgin pilots were talking about the fact LIFO had to be used - but it isn't law. Barrister Fenton Bresler in his book 'Law Without A Lawyer' states that:
"It is not enough for you to be made redundant. Your employer must also prove that they were fair in their selection of employees to be sacked. Selection criteria must be rigourously adhered to. A simple policy of 'Last in, first out' is not always enough. There must be a comparative, objective analysis of all the relevant information before a decision is made."
The recent ruling in the Rolls Royce case re-inforces this point.