What credit card do you use

Credit cards aren't as big in the UK as they are in the US, but I personally use Amex

Amex platinum and chase sapphire reserve. IMO those two are the best points ecosystems to get into young. 

For a lower level the Amex gold is a great card

Amex gold for everything. Just curious though -- at what point is it worth it to make the switch from Amex gold to Amex platinum?

Chase Sapphire Reserve

current lineup:

Amex gold for the 90k sign up bonus, cancelling once I leave the restriction period, chase sapphire pref, sofi card for 2.5 back everything. Last month main card was discovery for the 5% back on gas.

Chase Sapphire Reserve

Echoing others here, Chase Sapphire Reserve. It's just so good if you eat/go out frequently and have occasional travel. Mine pays for itself multiple times over. You can combine it with other Chase cards (Unlimited, Freedom, etc.) and then redeem those points on the CSR. 

I've looked into Amex Plat but I'm not sure if this makes sense for any junior finance professional. The Plat only seems useful if you're constantly booking flights and hotels (like consultants do).

Amex Gold and Amex Blue Cash preferred for me. I don't travel much so the gold is great for points back on restaurants, and the Blue cash card gives me 6% back on groceries and Gas, which is optimal. 

My main cards are Amex Platinum and Gold (combo for max points on travel and dining), amazon prime rewards card for 5% back at amazon and whole foods, and the American Airlines card to juice loyalty points by spend on categories I don't get a bonus on from other cards.