Unfortunately, there isn't really a postgrad converion course you can do from a history undergrad into STEM.
I've currently in the same boat, but from a different discipline.
You will need to either redo your A Levels (ideally) or choose another suitable Level 3 qualification (foundation year, Access, BTEC) to do your desired STEM. If it's in physics or chemistry, you're likely required to redo your A Levels or do a foundation year. If it's biology related, you should be able to do an Access or BTEC without issue. The most commonly required A Level to supplement Access or BTEC tend to be maths (particularly applicable if you're doing something heavy in physics, tech, and/or chemistry), so if you score high in maths and it's within the time limit your chosen university considers your A Level valid (can range from 2-5 years), then you should be good with an appropriate Access.
The PGDip or conversion courses tend to be for those who want to switch subjects within the discipline e.g. if you did an undergrad in engineering, the postgrad will help you get into physics; if you did an undergrad in biological sciences, the postgrad can get you into cancer research.
I wouldn't consider Pyschology to be part of STEM, but the conversion courses they offer tend to allow any graduate to go into the field.
For medicine, you could do an Access course, foundation year, or A Levels. If it's Access, then some unis are picky about which Access courses you do, and they tend to prefer those from colleges that seem closer to the uni. I am not entirely sure about dentistry, but since it's competitive, I wouldn't be surprised if they ask for something similar to medicine.
Other medical/healthcare related degrees tend to have funding available if you study on a part time basis. However, you might want to check on this for your particular course. Usually, the entry requirements are more or less the same as for biology related courses. Some might ask for chemistry related Level 3 qualifications.
Maths will require A Level Maths and Further Maths for some unis.
Engineering degrees tend to vary in requirements, but they should generally accept all types of Level 3 qualifications. Having said that, you can alternatively do an apprenticeship in engineering, if you can get it.
Computer related degrees tend to ask for A Levels. Funnily enough, A Level Maths tend to be the required subject and not computer science.
For some unis, you might also get entrance exams.
If you do intend to do an Access course to get into STEM, you should check whether the Access course you are doing has enough credits in the subjects the particular uni department asks for in the particular course you want to apply for, if the department accepts the course you are applying for at all. If not, they will likely ask for A Level Maths at the standard required grade.