Pressing "1" can have several effects
1) it can get your number flagged as being a potential viable target - number to be noted and maybe sold to other scam organisations
2) you get put onto a scammer - either real or automated - who will try to extract personal data from you "to confirm your identity" or "to confirm this is your account"
There are probably other options - but all of them could cause problems
Just a couple of notes -
Firstly - if the scam call advises you to ring a number - then DO NOT do so from the phone they rang unless you have confirmed the line has been disconnected. Scammers have been know to duplicate the dialing tone to give the impression that the line has hung up - then pretend to be the bank when you ring a number.
If in doubt ring another number, such as you mobile, from it to confirm the line is clear.
Secondly - report it to whoever you can. Action Fraud, TPS, the bank they are pretending to be and also whocalledme.co.uk
the more information there is out there the better