migrated from esa to uc due a forst paymen

UC is paid a month in arrears so you should be covered now.

However you won't have any proof until your first statement, which a chemist or dentist might ask for.

You can pay, request a receipt (FP57 for prescription) then claim back afterwards if necessary.

https://faq.nhsbsa.nhs.uk/knowledgebase/article/KA-03917/en-us

For a prescription just take the receipt and FP57 into any pharmacy. If the refund form doesn't have a UC box, tick JSA.

Free prescriptions aren't automatic with UC, check you meet the criteria here:

https://www.nhsbsa.nhs.uk/check-if-youre-eligible-help/getting-benefits/hwhc-universal-credit

Take home pay refers to money earned through working.

You tick box U Universal Credit, if there is no box U then you tick box K JSA.