...that...doesn't strike me as a fantastic trade. The RP-1000, quite apart from not being all that great in the first place, retails for £100 *less* than the theremin does.
You'd be trading a high quality, albeit niche, piece of expensive kit from a respected high end manufaturer, for a mass produced, not-exactly-well-regarded piece of kit from a low tier manufacturer, and you'd be adding insult to injury by being basically £100 worse off in terms of monetary value.
I'd sell the theremin via another route and buy something more worthwhile like a BOSS GT-10 or POD500. They're more expensive than the theremin but they're both going to make the RP-1000 look very much like the terrible idea it is.
Here's a comparison on thomann
http://www.thomann.de/gb/digitech_rp_1000.htm
http://www.thomann.de/gb/etherwave_theremin.htm