It seems odd that a room where you spend so little time when you’re at home can involve so much expense per square metre and frustration in the building phase.
When a builder is pricing a job they’ll give you a keen price, which will often mean they are assuming the most straightforward of installations. A move away from those assumptions after the price is fixed can be costly – either in terms of relationship with your builder or your pocket.
If you go direct labour and hire the trades yourself, you will need to line them all up – general labour but also carpenters/joiners, plumbers, electricians, tilers, plasterers, decorators, and make sure they all know what is being asked of them and that they agree with your plans and choice of fittings.
As with your builder, make sure you agree in advance what each of the trades will and will not do – not all plumbers will run the soil pipes, for example.
Bear in mind some plumbers may be used to certain fixtures/fittings and may not be comfortable fitting certain types of gullies or shower trays for example. This could have to do with the type of warranty or backup service they provide.