I made a system for making more ring power, and that will also fit the UK-system. The relay will almost draw nothing from the line, and the output to the third wire will not be affect the incoming ring power.
For UK I would suggest to add e 1.8 uF capacitor in series with the ring line, just because of the characteristics of the AC inverter.
http://www.classicrotaryphones.com/f...9848#msg229848
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