Speaking from experience, Marconi, Nortel helped a small startup company in Oxfordshire (we bought both Nortel and Marconi). That startup is now based in the USA and the old Marconi plant is no longer, not sure about the Nortel plant in Paignton.
Even back then the largest client for optoelectronics was Huawei, but you don't need to compromise the switch, you can quite easily tap directly into the fibre connect at the point where the amplifiers are based. You may think that fibre can transmit to some near infinite distance but there is an actual physical limit to how far it can go before you need to boost the signal again. At this point you can then take a small tap, ostensibly to check the signal strength prior to boosting, but there's no reason why you couldn't redirect a copy of the signal to any intelligence service at that point. All completely invisible to the rest of the network.