Re: termostatic valve
Not sure about Baxi makes and models we don't use them but on the boilers I have used the mixing / blending valve is on the return, working back from the boiler you have the pump then the mixing / blending valve which is teed to the flow. The purpose is to maintain the return at a set temperature to the boiler to prevent condensation which shortens the life of the boiler. There are other valves so its important not to confuse them, for example there is an automatic mixing valve used to protect the pump especially on zoned systems. As a zone closes the valve opens to allow the pumped water to return to the boiler without putting the pump under pressure causing the pump to "dead head" basically pump against a closed circuit. I expect your plumber has the pump working away from the boiler in the direction of the flow. There can be a few reasons for this so your plumber is not necessarily wrong he may have installed the boiler that best suits your heating system.