Oh no! The OP reminds me of me.
In fairness, I'm going to have to agree with the others: leave it for now. If you explain to the installer what you want to heat and that you want to be able to control the bathroom manually, I'm sure s/he will be able to accommodate your personal preferences and, if that is the bypass radiator, another radiator could possibly become the bypass radiator instead. It may be that one of those valves on the radiator is a manual valve you can use to shut it down, but best check as sometimes things are left a bit weirdly when a system is under construction.
If you're worried about wasting gas, here's the plan for the weekend: turn the rads on in the rooms you want to heat (to max or 5), and turn the heating off when these rooms are all hot enough. The room that heats up last you leave on 5, the other rooms you turn down to whatever setting keeps them the desired temperature while you control the room that is set to 5 by turning the heating on and off.
The above means the bathroom radiator only comes on when the heating is actually switched on, which will save something, and you'll be doing the job of a room thermostat. Stupid, but if it's stupid and it works, then it isn't stupid.
If that bathroom rad is a 1kW radiator, it's probably only costing you 5p an hour anyway (for the bathroom rad), so even if it stays on constant all weekend, it's less than the cost of a Sunday newspaper. Environmentally, it's the same CO2 as driving 38 miles, and if money is tight it's tight, so I do see where you are coming from.
If it makes you feel any better, 38 miles is almost exactly the mileage I saved earlier this evening by catching the train instead of driving.