The cheapest (if this equipment isn't too valuable) would probably just be to bubble-wrap it really well and send it via Royal Mail. Recorded delivery is of course safest but slower than regular post (at least on the Polish side).
As far as I know, all envelopes (whatever size) go by air to the uk. I usually put one padded envelope inside another, get it registered and marked 'priority' and Bob's your uncle. It arrives a few days later.
The catch is to make sure it's an envelope, otherwise it's a parcel and takes longer.