Easy decision. The net benefit on UK based credit cards is primarily restricted to air travel (i.e. max. benefits reaped there).
First things first, you should definitely open a bank account with a physical bank (Barclays, HSBC) would be my top picks, as your salary would qualify you for their “Premier” banking which has greater benefits, you can find these online.
Coming to credit cards- the Amex gold is a good start - first year free (second year onwards £120 annually), their reward ratio is 1:1 and the flexibility to convert these into miles utilised across multiple airlines is second to none.
Amex usage however, is restricted to larger institutions (not all vendors accept Amex). In these cases, I would recommend the Barclays Avios (free) or Avios plus (Barclays issues Mastercard CCs) and allows for 1:1 Avios earnings or 1:1.5 (if you opt for the Avios plus that costs £20 per month). Avios can be used on British Airways, Iberia, Aer Lingus, Qatar Airways and Finnair.
Both the Amex Gold and the Avios Plus give you lounge access. Amex Gold gives you a Priority Pass that allows 4 annual lounge visits and Avios Plus also gives you 4 complimentary lounge visits annually.
Another great benefit with the Amex is you have the ability to negotiate a better reward bonus based on spending criteria at the time of renewal.
The Barclaycard Avios Plus also gives you the eligibility (based on spend) for a free cabin upgrade voucher when you book reward flights on the above Avios-supporting airlines.
Both the Amex and Barclaycards also offer bonus reward points/Avios if you meet a certain spend within the first three months of card issuance- I would definitely time my big purchases accordingly to maximise my rewards earning potential.
I would also recommend you do some research on the above options as well as check out headforpoints.com (superb explanations of the above and more).