If I only want to pressurise central heating not hot water should I get a regular boiler or system boiler
I was in the boiler business for 40 ears and am having a hard time understanding your question. Here in the USA we almost always, install pressurized hot water heating systems. I do not understand the terms you are using, "system boiler' or "regular boiler". You also said that your system is a regular open-vented system. We had systems we call open or vented long before I started in the profession; (the expansion tank was an open and vented tank, mounted at the high point of the system with a drain line that ran to the basement). Sometimes, it is hard to explain what we each see when different countries explain things differently. No offense intended.