You really can't - you get the same issue with silicone. It's just not worth the hassle. Your best bet is to hit Harbor Freight or Home Depot and buy a bag of cheap disposable brushes. I think the last bag I picked up was around 20 assorted brushes (1" - 2") for like $6-$7. Don't waste expensive paint-grade brushes on stuff like this...
*EDIT* Sorry - I just saw that you are in the UK. I'm not sure what's available but there ought to be some equivalent product local to you...
Four brushes? That's nothing. Expect to use dozens more. They're meant to be used only once. Once the resin starts curing (or harden), the brush is done for. Some might suggest soaking it in MEK (methyl ethyl ketone) but IMO, it's not worth it.
However, if you used 4 brushes just to paint resin on a small area, either you're using too much hardener or that you're working too slowly. On average, I end up using 2-3 brushes to put a single coat of resin on a helmet.
As xxFemale Spartanxx said, acetone is your solution. Any hardware store has it and it is a main ingredient in most nail polish removers. Be careful as to what container you use to keep it in though, the resin it eats off will still melt plastic cups and what not.
I kept mine in a butter tub actually. What I did was drop the brush in it when I was done using it and I would keep a bit of aluminum foil over the whole thing. You could do what I did eventually after I got a little smarter. I kept a smaller butter tub inside a much larger one that had a lid. The smaller inside tub held the acetone, the outter one kept it from evaporating. When you are done with the brush, stick it in the acetone for a while. When its been in there for like an hour take a napkin and brush the brush along it to clean out most of the excess resin. When youare done go ahead and rinse it off. When it dries you will be able to use it again.
Problem is brushes are only good for 2 maybe 3 times then they are trash.
It never did eat through the butter tub. Eventually you will get a lot of gelly in the bottom of the butter tub. Take a napkin and pour it out over the napkin, make sure you are wearing gloves. With any luck the napkin will catch all the jelly resin and the acetone will run off onto the floor of your garage or wherever you are at. Refill the butter tub with new acetone and you are good to go.
I've had a lot of pep success in the past with those black foam brushes. They don't perform as well as actual bristle brushes, but they are super cheap and you can get about 20 of em for a buck or two. Only thing you have to be careful of is that the resin will dissolve these brushes with a quickness.