So you can't drill the steel H section in case you weaken it but you can drill a 25mm hole in every wooden floor joist supported by it without any worries !!
Wooden floor joists holes are covered by a standard rule defined, I believe, in the original wood joist span tables which used to be published in the building reg approved documents. You can only drill along a certain section of the span though.
Steel beams though are usually only included in a building after a qualified designer has performed the appropriate load calculations, and there are no pre-determined rules of thumb defined in the approved documents.
So, whilst it may be perfectly OK in practice to perform your own estimate of the effect of drilling a hole, the general consensus is that the cost of drilling this hole legally (with paid-for calculations) is much higher that simply finding a better route.
