What are the immigration procedures when entering the UK with both an old and new passport after applying for a visa?

Highly unlikely you’ll get a Visa issued in 9 days (how many working days?) I recently got a new passport in the Uk - same day but had to book an appointment at the passport office in East London and pay a premium.

The old passport is always chopped. You can't have two valid passports. A new one isn't issues until the old one is in their receipt.

Wait two days then apply with new passport. why the rush, you scared in case yer bird finds someone else whilst your here

Think you would be very lucky to get the evisa in 9 days

Why don't you just WAIT to actually physically RECEIVE your new passport, then, afterwards, apply for any visa with any country's respective Embassy/Consulate, or, via their Embassy/Consulate Visa Website??

Do not know the specific details of this "old" passport, but if it has only a couple months before its EXPIRATION, then no visa will be issued by most countries Embassies/Consulates and that visa application on the "old" Passport could be cancelled/rejected

Also, if this "old" Passport is full and has run out of empty passport pages for any visa and any immigration entry+exit stamps, then any country's immigration officer at arrival may not permit entry

in both examples cited above about "old" passports, the airline check–in counter staff may not even allow check–in nor issue a Boarding Pass, even though you already bought a plane ticket as that "old" passport may not have enough validity (almost expired) nor enough empty passport pages that they (airline policy) would deem to make a potential passenger boarding their airplane's flight NOT be permitted entry at the destination and that potential passenger needs to be returned back to the place of departure and the airline gets fined by the destination country's Civil Aviation Authority for boarding a passenger when they should not have accepted a passenger with this "old" passport

You can enter without any issue providing you have both passports to show them old and new,

I occasionally travel with 2 valid and in date UK passports, one almost full and one 50% full, I asked them last year on arrival at airport to transfer my extension multi reentry stamp from one PP to the other and they obliged no issue whatsoever,

Having a new passport doesn’t change your travel history as everything is electronically linked

Visas issued outside Thailand and you don’t have a extension yet, will not be transferred to your new passport but they will remain valid.

You have to travel with both passports.

When you are applying for a 1 year extension inside Thailand all the information will be transferred to your new passports before you apply for the extension.

What sort of visa are you applying for to be confident it will be processed and approved in such a short time?