Are these the things that do the Brighton to London Bridge run?
If so, I use them occassionally and there is nothing that wrong with them. No reason why they couldn't go straight into service up here. They are vastly better than what we have today.
Depends on which you mean. FCC and Southern operate Class 319s, which are like 4-car electric Class 150s, built for Thameslink in the mid/late 1980s. Most have had one or two refurbishments already, the FCC ones quite recently (since 2006, at Hornsey). They are not-DDA compliant, so would need costly refurbishment prior to 2020 (or later, if the law is relaxed).
They also both operate Class 377 Electrostars, which are like 4-car electric Class 170s, introduced recently as part of the start of the Thameslink Programme, but will be replaced by more new-build stock later.
Another big issue is that they are designed for Driver Only Operation, whereas we operate with the Conductor/Guard controlling door operation. So modifications (wiring, etc.) will have to happen, or the unions will kick up a fuss.