It's been a while since I ordered any, but I think that 4350 is about £100 for a single 18" x 12" sheet of plain material. 4003C is a lot more, or it might be the other way round.
I've generally gone down the route of using expensive but trustworthy board houses, as delaying a project by a month due to a crap board spin costs way way more than even a very expensive board.
Your price are a bit high. RO4003C is about $100 (not GBP100) for that kind of size, RO4530B is about half that. I have only briefly looked through RFMW stock. I do not know where else I could buy these.
I am not afraid of the Chinese guys screwing up, they actually never did any of my PCBs. But the expensive local mfgs, yes they screwed a lot. Hair, someone's oily finger in a soldermask, inconsistent solder mask color, ENIG board that had a leakage due to gold depositing in a microfilm all over the board, not abiding to the extra cost express manufacturing deadlines, and such. The chinese guys can screw you in many different ways, but from my experience the failure rate there is much lower actually. But I am only mainly a hobbyist, so I do not spin off that much PCBs.
G0ZHU: 20mil substrate seems too thin, I understand it may be very fragile. I'd rather use something in the range of 0.5 to 0.8mm, for example the 30mil or 20mil RO4530B substrate, as it gives reasonable 50ohm trace width I am used to.
I do not have PCB mill, my friend does, but I would not trust that for precision. And it is tiringly slow. But I have no issues whatsovever etching my our PCBs using the photographic method, with as good precision as it can get at home. (600DPI laser printer) Can do both positive and negative solder resist film.
Problem is, where to obtain (in EU) the substrate. I only mainly find RT/duroid material, which I know nothing about. Only that HAMs used that a lot in the past, as there wasn't anything much different available, apart from plain very thin PTFE laminates, which are pain in the butt to work with. Do you have any ideas, where to buy some in the EU, especially RO4530B substrate in about a the 0.5 to 0.8mm thickness range, without the cost being stupidly excessive due to "exclusivity" of the goods?
However doing such RF PCBs at home has major drawbacks and that is no plated vias. I am used to work with small SMT stuff, including fine pitch QFN (which most RF/uW chips are these days in anyway), where it is mandatory to use small vias and vias under the devices itself (exposed pad GNDs and such). Hence looking for a cheap manufacturer, that can do the 4530B or 4003C. (Guessing, the 4530B should really be the cheaper and suitable option for a home-gamer. Still huge leap better than FR4!)
I do not think the price is high due to increased precision expectancy, for example in the Bulgarian manufacturer, one can clearly specify the precision, yet the price is a nonsense. The high price is I think mainly due to exclusivity. The vendors think, they can just simply rip off people, as corporates almost don't give a sh!t about the price (they just want the work done) and home-gamers are only a small if not almost zero fraction of the customer base for such special materials.
//EDIT: Even one of the chinese guys I have asked for 4003C gave estimate $300 for a small, 45x30mm pcb x 5pcs, with almost no drilling, no solder mask and silk, ENIG. I was like WTF?!