The UK laws are quite relaxed.
Some info here.
One thing you will need to note before you get any labels made up is to not put 'Made in England', unless the tee itself was actually sewn together in England.
Printing and relabelling does not qualify a garment Country of Origin status.
While you do not need to put a Country of Origin on the label for the UK and some EU countries (unless your tee has a flag or the like on it), you
will have to if you are shipping to the U.S.
More info here at Section 6 - Chapter 61.