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For more on marking an answer as the "Best Answer", please visit our FAQ.I'd be amazed too if there's no drain valve anywhere. In one new house I owned it was to be found right under the backdoor step (house with all 15mm copper pipes into the concrete screed). In another new house with a 22mm pipework system running to a microbore manifold, it was incoroprating as part of the manifold, which was in a downstairs cupboard. You could try both of those places.
If you still can't find it and don't fancy the (good) idea above, you can drain the system by selecting one downstairs rad, closing off the valves both ends (lockshield and thermostatic) then releasing one of the nuts attached to the rad itslef. Be ready to catch about 3 litres of water which will come out of the rad itself. I use empty icecream square containers - ideal. Then undo the other end and remove the rad itself (small bit of water to be caught). Once the rad is off, you can open the lockshield up gradually and let the whole of the system water out in a controlled manner. I've done this with a fitted carpet in place and avoided upsetting she who must be obeyed with any mess.