Did some quick maths and with the weights available at my gym:
Two 1kg plates
Two 2kg plates
Multiple 1.25kg and 2.5kg plates

I can get every 0.5kg increment except 0.5, 3, 15.5 and 18 (I think).

10 = 10
10.5 = X
11 = 5 + 4 + 2
11.5 = 5 + 4 + 2.5
12 = 10 + 2
13 = X
13.5 = 5 + 4 + 2.5 + 2
14 = 10 + 4
14.5 = 10 + 2.5 + 2
15 = 10 + 5

Then add 5 kg to each of those for 15 - 20.

This realisation saved me from having to buy lighter plates online and bringing them to the gym to up the weights by 0.5 or 1kg.

Thought it was interesting and might be helpful to some people without access to lighter plates to microload.