Ohh well, in for a penny in for a pound - I am new to this forum - joined last night, but am finding it quite stimulating and thought I would give my opinion on your particular situation - so here goes:-
In basic terms this type of stud wall is a structural element and not simply an infill panel to divide off two areas and acts to support the ceilings joists which rest upon it.
The diagonals themselves are braces and the stud wall is a form of construction regularly used in the year dot as it was a cheaper alternative to brickwork, the diagonal braces act to keep the stud wall aligned/upright.
The stud wall was used as the load from the loft space would be far less than that associated with occupied rooms and therefore the wall does not need to have the load bearing characteristics of a brick wall and is quite capable of taking the load from the ceiling joists (and originally the weight associated with a domestic water storage tank, which would have been located in the loft space.)
The studs themselves are spaced such that the load from the ceiling structure above is transferred as a distributed load to the brickwork below.
Insofar as running any pipework - assuming the incoming mains is a 1/2" (12.5mm) diameter pipe there would be no harm in drilling say a vertical 5/8" (15mm) hole down through the centre line (neutral plane) of the diagonal brace and run your pipe, this option would leave sufficient meat on the timber brace not to have any detrimental affect on the functioning of the brace.
One word of caution, do not think about removing any of the studs at a future date without getting some professional advice!
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