Earlier quoted context omitted.
A substantial chunk of ESA's budget comes through EU. So the cost of UK's membership in ESA might increase, as they won't be participating through EU anymore. That's the kind of thing that is hard to justify to Brexiteer voters though... "Now that we're rid of the EU, we have a unique opportunity to pay more tax money to other intergovernmental organisations!"
Here you have ESA funding by source: http://www.esa.int/spaceinimages/Images/2016/01/ESA_budget_2... EU funding doesn't go to the main funding budget of ESA but to other activities like the Galileo satellite system.
http://www.esa.int/About_Us/Welcome_to_ESA/ESA_and_the_EU
"Some 20 per cent of the funds managed by ESA now originate from the EU budget."
I'm not disagreeing with you, but it seems unlikely that UK just won't participate in that 20% of ESA activities anymore. So they'll have to renegotiate the membership fee to account for that.