There was a 2018 meta review of antidepressants that got a lot of media attention at the time: it was supposed to be proved that they do work better than placebo.
Anyway, from memory, Escitalopram came out as the best SSRI taking into account both tolerability and effectiveness. However, it's a matter of individual response, so you just have to try it vs Sertraline to know for sure.
Citalopram just is Escitalopram in one sense, but in another sense they are different. Mirror images is kind of it, but Citalopram is a mix of two stereoisomers (one of which is the Escitalopram form), whereas Escitalopram just has that one form by itself.
So 20mg of Citalopram contains, in a sense, 10mg of Escitalopram. Are they really any different to take? Possibly they are, as it is theorized that the other stereoisomer isn't merely inert, but actually gets in the way of the active form working.
As for price, a few years back Escitalopram was twenty times the price of Citalopram, so understandably it may have been harder to get on the NHS. The price came down to about the same, but has now gone up a little bit. Nevertheless, it's still today a cheap drug for the NHS and if you are paying a prescription charge the cost is likely completely covered. I don't know a reason today why you wouldn't use escitalopram in favour of citalopram. Less QT interval prolongation also, which is a minor concern in using them.