The job is replacing a new shower tray

no it is not illegal.

It is legal, but it tends to be suspisious. 25% its more ideal. In my opinion.

I would normally ask for the cost of any large fittings, such as a shower tray, up front - not usually 50% though

His terms can be whatever he likes, the same as yours, if you don’t like his terms you are not obliged to agree to them and can go somewhere else.

it isn't illegal, he/she is setting out his/her terms of the contract before the performance, the other party can either accept, decline or make a counter offer. What may be considered unacceptable would be if these terms were changed after the agreement was accepted by both parties and during the performance eg. the plumber has turned up on day 1 and asked for an extra 10% advance.