You UC payment will likely increase, and they should see how much you’ve been paid via HMRC so you don’t have to tell them (but I’d double check your statement to make sure it’s correct)
And it will increase also
You should be able to tell from current statements. It should say what your entitlement is, then what you’re currently deducted due to SMP.
They know as your employer will send them your earnings of £0 it’s all automatically done.