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Why you using 2.5 with a 16a? Could use 1.5.
2.5mm gives more flexibility with regards to cable routing and distance than 1.5mm due to it's reduced voltage drop (greater distance for a given design load) and overall larger current carrying capacity (2.5mm can carry the design load of 16A for all but one listed installation method).
Appendix 17 of the BBB also makes reduction of energy losses in the electrical installation a consideration for the design of installations. Given an ASHP is going to be running for a long time, the choice of 2.5mm will help reduce running costs by reducing losses in the cable. 1.5mm drops 29mV/m/A, 2.5mm drops just over half that at 18mV/m/A, so for a given cable length it will reduce needless loses by just under 40%. Over the life of the installation, that could add up.