You cannot use your US MESA in the EU/UK, as you must use an appropriate child seat for the specific market that conforms to the United Nations standard, ECE Regulation 44/03 or ECE 44/04, or is approved under R129 (also known as i-SIZE). The seat must also be suitable for the child’s height and weight and is correctly fitted according to the manufacturer’s instructions. Only an EU/UK approved car seat can be used in the EU/UK. Child car seats approved outside of the EU/UK cannot be used in the EU/UK.
Similarly, you cannot use the MESA i-SIZE for travel in the US or Canada as you must use a seat that meets FMVSS213 and CAVSS213 regulations for car seats.