Hi OP, I'm Manx (currently working & living in England), and I'll help if I can. I've always said that if I ever get married to my long-term partner it will have to be back home in Castle Rushen! Beautiful place, opposite the old police station with the funky turret, round the corner from the George Hotel for a fantastic meal & accommodation. He's now seriously looking ... he's like, 'why be brought up round the corner from an actual Castle and not get married in it?'
I don't actually know anyone who got married or attended a ceremony in Peel registry office! They've all been in churches and Castle Rushen. (Can you tell which bit of the Island I'm from?!) You could maybe ask on one of the Manx Facebook sites? Or email the registry office direct - Manx people are usually pretty helpful. Or a poster called ManxExile on here might know. I've seen other manxies here, too.
Will the wedding party all be travelling over from the UK? Are you familiar with the Isle of Man at all, and its different laws? Budget? Will you have transport while on the Island?
The weather can put the mockers on the best-laid plans btw - my cousin's was almost cancelled because of this when the groom's family were stuck in Heysham! - and don't expect your mobile phones to work
Also: think parking. Some times of year can be very congested, e.g. bike race weeks, although given your wedding party is small it might not be too bad. Peel Castle has plenty of parking along the breakwater side (not so much fun though in strapless silk and heels in a 70mph hooley), and the House of Manannan is nearby, back along the quayside and opposite. Both lovely places, but I prefer Peel Castle for its stark beauty and vibes.
If you want to use heritage transport as part of the day, then you could use the steam train which stops in Castletown, but doesn't go to Peel. It's also seasonal, i.e. closes in winter season.
Happy to chat.