As others have said it won’t hurt to ask but there won’t really be much negotiation.
The market supplement does show that you have a desirable specialism, but I’m aware of a department that does have the standard grade pay scales plus a supplement to retain specialists / particular roles.
You will still have bank holidays to add, plus as some have mentioned potential privilege days which can slightly differ between departments
I would expect you to get 25 days leave, plus bank holidays plus 1 privilege day as the sovereigns birthday.
The 25 rising to 30 over 5 years is likely to be one extra day per year
Historically the civil service had 2.5 privilege days which government removed, but the Queen pointed out that the sovereigns day was granted by the king rather than a government term of service.
I believe that there are one or two job types that still have 2.5 privilege days, but they are a rarity.
For me I have ‘lost’ 1.5 of the privilege days, but they are in the terms of my contract due to when I started so I instead have 31.5 days leave + BHs + 1 PD
(That means I get to choose when to use my 1.5 as leave rather than the set periods when they were privilege days)
If I take promotion again or transfer department then my contract will change and I’ll lose the 1.5 days
You of course have nothing to lose over asking for going straight to 30 days (unless you don’t accept the offer, saying you want 30 days or don’t join and they then say no)