Yes, and definitely not regretted it. The demands of UK academia today are such that I do not envy anyone continuing to work full-time into their 60s.
Some of the answers depend on your employer's specific procedures. Pension? Don't know whether you are in USS or TPS - in either case the best way to find out pension implications is to phone their helpline. But I would be wary of approaching HR just to find what your salary would be (probably unnecessary anyway for that information, as this sounds like a straightforward 20% salary reduction; you can calculate your new take home pay using MoneySavingExpert's Income Tax Calculator; and may be pleasantly surprised, as for tax reasons the take home pay reduction will be less than 20%, and you can also subtract from the reduction what you'll save on commuting costs). Better to speak to a trusted part-time colleague or union rep.
Your employer is obliged to consider your request, but not to accept it - however in the current perilous financial climate for universities is likely to welcome saving 20% on your salary and employer's pension contributions. When you make your request to your line manager, present an itemised list of demands so as to get clear agreement on which specific 20% of your job roles you will no longer do. You are in a good position to negotiate, as from your employer's point of view you are offering to make 20% of yourself redundant - what's not to like! But especially if the details are likely to be tricky, seek advice from a trusted third party and exercise your right (if you have one) for you to have a supporter attend the meeting as an observer.
Once you've gone down to 4 days, be firm about it - this is not as hard as it sounds, as being part-time is a near-unanswerable way to say no to requests to take on additional unpaid tasks ('Sorry I can't do that because I am officially contracted to do 20% less' trumps 'Sorry I can't do that because I've got too much on at the moment'). Yes there will probably still be occasions (e.g. marking season) when in practice you won't be able to keep to the hours you are paid for, but it is better for your wellbeing to be paid 4 days for sometimes working 5 than be paid 5 days for often working 6.
Mondays vs Fridays? Depends on your circumstances: both have their appeal. On balance Mondays gives more of a psychological boost given that many people dread Mondays, especially if you are in a workplace where many fulltime colleagues disappear on Fridays anyway, so you could feel cheated if that is your only official weekday off. But Wednesdays, if that's when meetings happen, could be best so you can get out of meetings and have a well-earned breather midweek. Annual leave? Complicated as for part time workers it's normally calculated in hours rather than days, so best to ask a colleague at your university who is already part time. But above all enjoy that whole extra day off every single week! Hope that helps - best of luck.