Is it possible to implement MITM attack on larger networks than LAN? For example is is possible to implement such an attack on the whole network of an ISP?
If you are said ISP or government, then yes it is possible. GCHQ surely has the capabilities to intercept huge chunks of Internet traffic. They are most likely able to do some amount of MITM but it's not been disclosed whether they do that or not as part of any program.
I would also assume that a competent adversary could break into an ISP and target a specific client of that ISP, though I've not heard of such attacks in years.