However in reality, the UK is tapping all undersea cables coming into the UK (so basically almost all transatlantic ones), and logging everything. https://www.theguardian.com/uk/2013/jun/21/gchq-cables-secre.... This is done under the Tempora program: https://en.wikipedia.org/wiki/Tempora.
So any connections into the UK are also being logged. This map gives you idea of the scale of connections coing through the UK: http://submarine-cable-map-2016.telegeography.com/ and this map shows the taps that have been discovered as part of the NSA Snowden leaks: http://lifewinning.com/submarine-cable-taps/ (Source code: https://github.com/lifewinning/submarine-cable-taps)