gottaa - any club that refuses to let you use scratchbuilt *ork* vehicles because they're not 'proper games workshop' clearly is missing the point. hell, theres a bunch of things you cant get in proper games workshop at the moment - some tyranid stuff doesnt exist, the BA Stormraven (assuming its real) doesnt exist, for a long while some of the ork stuff didnt exist...
GWs problem, for tournaments at least, is that they dont want copyright issues if they put some other company's models in white dwarf in tournament pictures. so they dont allow any custom stuff in tourneys. and this attitude tends to filter down to some clubs, i guess.
when they say 'no forgeworld stuff!' it normally refers to forgeworld *rulesets* rather than physical models. For example, there are forgeworld rules like for Hazard Suits -
http://www.forgeworld.co.uk/pdf/xv9.pdf - essentially a step up from Crisis suits (and, imo, lovely models
http://www.forgeworld.co.uk/acatalog/xv9store.jpg ). So using them would be a nono, because theres no equivalent to them in 'official' GW rules. otoh, the standard GW broadside models are a bag of arse, and the forgeworld ones are lovely
http://www.forgeworld.co.uk/acatalog/bsidep2store.jpg , for example, so i'd expect them to allow the use of those, unless they're being over-restrictive just for the fun of it.
Forgeworld is a part of GW, their models are official, but their rulesets are designed for more self-regulated games (like apocalypse) and therefore might not be permitted in 'standard' games.
the reason for this isnt really that small models (like the hazards mentioned above) would be out of context, its that if you allow those, then you open the door for people spending 900 points of a 1500 pt army on a Thunderhawk gunship, that is essentially unkillable in a game of that size
uh, that was a bit of a rant and i probably repeated myself, oh well
gist of it is:
custom models should be fine for club play, custom rules are not.