Spending Car Allowance

I’ve recently been offered a new job at a different company at £30k p/a plus a £400 p/m allowance for a car.

My current role is the same salary of £30k p/a but with a company car (or my company calls it a - jobs need car) (which includes a fuel card, as I require both to undertake the role). And works out at £5293 as the value of the benefit.

My new role won’t require me to use a car for business use as my travel will be to London via train which will be covered by the company. My car allowance was thrown in as it was important for me to still have use of a vehicle (me and my wife both used the company car for personal use). I understand the company not offering me a company car given the fact they’ll be paying for my train travel already, and a car for business use won’t be nessasary.

What I’d like to know is firstly how to I spend the allowance? Am
I after a PCP or PCH plan?

Do I go into a dealership and tell them it’s a car allowance?

I’ve financed a personal car before but was a long time ago. Would I need my own deposit on a vehicle? Etc

I understand how my tax is calculated for my company car, but what changes with an allowance and how do I calculate any tax implications?

A lot of the forums talk about people having the option of either company car or car allowance for cost effectiveness, I don’t have that option and will be returning my company car for my currently company when I leave so will be taking this car allowance regardless, and hope I am not worse off financially than my current role! (If someone can advise on that point too that would be great )

Thanks