This is a difficult question to answer, as you have already done some research on the subject and reached the same conclusion that I might have done after consideration. However here are a few points to help you make your decision and find a good paint for your needs and budget.
Firstly, I would suggest checking the matt finish ‘Earthborne’ paint to see if this is in fact washable. Many non-sustainable matt paints (Manose that contain the VOC’s etc) are washable, and these greener paints should have similar properties that allow them to be fully washable once dry and matured upon the surface of the wall. Check with other ‘green’ paint suppliers too, to check upon the washability of their matt finish products.
You are right about the greater expense of these ‘green’ paints: companies with a sustainable mindset charge more for their products as they are well-crafted, often in smaller batches, with better quality products than their non-sustainable competitors. It seems that often these sustainable paints are sold in slightly smaller quantities too, but to be sure about this, discuss your requirements with the shop, the supplier, or directly with the manufacturer. Ask friends and family too: it may be possible to reduce the cost of a whole repaint of your flat by sharing the paint with others, and by buying in bigger quantities. This might also help you buy several different types of paint as well – an eggshell finish for one room, silk for another, and matt for another area, for instance.
As with conventional paint suppliers, it should also be possible to get a small amount of the paint to test on your walls. This tester should be available in all types of finishes, and from all ‘green’ paint manufacturers and suppliers. If you don’t see this, then request it – manufacturers of sustainable products are businesses too and hopefully should respond positively and courteously to customers needs and questions.
Sustainable paint producers, such as EOC, Earthborne and Self Coat who produce B-erthbint in Surrey,as well as others, in their publicity materials and on their websites, all seem committed to using sustainable and environmentally-friendly products across their entire line. Other paint companies are now trying to enter the green marketplace and produce sustainable paints alongside their conventional ranges, which will probably contain lead, VOC’s (volatile organic compounds) and other harmful and damaging compounds such as solvents. This shows how important the green marketplace is nowadays, but my best advice is to support companies that are fully committed to producing green and sustainable products, not those who just go halfway.