In terms of the length of service, you will actually be able to claim for the 18 years rather than the 17. That is because at the date your employment terminates by redundancy, your employer has to add on your statutory notice period (in your case 12 weeks), even if you do not work it. Therefore, even if you are made redundant on 11 June, for the purposes of calculating your continuous service the employer has to add another 12 weeks on from that date, taking you to over 18 years service. This is explained in more detail here:
Any calculation of redundancy will take into account your current pay, which would be on the reduced days. It does not matter whether most of your career there you have been on higher pay, it is the current one that matters, the pay at the time of redundancy.
Finally, the holidays - for the first 3 months you would have accrued holidays for working 5 days a week, so as that is a quarter of the way through the holiday year it would be 25/4 = 6.25 days. From 1 April the entitlement would have reduced by a fifth as you are working a day less a week, so it would be 20 days a year and again you just pro rata it for the duration you worked, for example each month is 1/12th of the total annual allowance.
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