The government
Maybe not the government that rules the country you live in, but a government anyway. It’s illegal for the NSA to spy on American citizens (not that that’s ever really stopped them), just like it’s illegal for British foreign intelligence agencies to spy on their citizens. It is not, technically, illegal for British intelligence to spy on American citizens and then tell the NSA what they heard, and vice versa.
Government agencies—especially spy agencies—love having “deniability.” Deniability is great. It’s when everyone knows you’ve done something bad, but no one can prove it.
When you capture an enemy soldier, you might move him around, passing him through the hands of various agencies from allied countries. You might end up leaving him alone for a few days in a secret Polish prison under the care of, let’s say, Turkish secret police. If, when your captive is handed back over to you, he is missing some teeth or fingernails and accompanied by a dossier of information on enemy troop movements, who are you to say what happened? If he was tortured, it wasn’t your fault! You weren’t even there! Blame the Turks! Deniability.
The same thing goes for domestic spying. When you have an intelligence-sharing agreement and share an apparatus like ECHELON with other countries, it’s entirely plausible that there may be a tacit agreement to spy on each others’ citizens and hand over regular reports. There’s no reason to say where the information came from, only that it may be relevant to everyone’s security.
The NSA has displayed little shyness in recent years about illegally spying on American citizens. But before that, it probably just got its information on us from England.