What should I do if my TR visa is issued after I arrive in Thailand?

You left it a bit late as probably the embassy are on a oublic holiday for Friday and monday

If you arrive on a tourist stamp and it arrives after, you can exit and return on that evisa (within 90 days of it being issued).

So in theory you can arrive visa exempt (if you qualify) 45 days plus extend for 30 days once for a total of 75 days.

Exit (by air or land)and enter on that visa for 60 days and then extend once for 30 days - total 90 days.

Added together that is 165 days.

And then you can exit and do the first step all over again....

You cannot activate a visa inside the country, only on entry. It might even work out better for you if you are trying to maximize your time as you could do 45 day visa exempt + 30 day extension, then border bounce for the 60 day tourist visa + 30 day extension.

That being said, London is known as one of the fastest consulates for e-visa and averages 2-3 days from the time of completed application to receiving it in e-mail.