Other London airports

Having used Stansted, London City, and Gatwick, I don't think any of them are any easier as far as getting through immigration, customs, and security. They are all pretty busy. The only reason for choosing these airports over LHR or London City would be if the flights were more convenient and more budget-friendly. (thinking Ryan Air and Easyjet).

For general ease of use and location (assuming you want to be in central London) LCY is head and shoulders above all the others. But, because of that, the flight prices may well be more than other less convenient airports like STN and LTN.

Luton is getting better all the time and is now easily reached
Gatwick is just like it's always been - bit stodgy but has some good routes
Stansted goes to a few different places, mainly due to Ryan Air but don't try to go in and out too early or late
City is a great airport but doesn't go many places and can be a little more expensive as the planes are smaller
Southend can be forgotten unless you are from Southend

Actually Southend has just got scheduled flights back with Easyjet. It also has it's own railway station steps away from the terminal, with very frequent service on the Southend Victoria to London Liverpool Street line.
It's a great wee airport. If and when it gets more scheduled flights it is very worthy of consideration.
The other airports I would rank as LCY, LTN, GTW then STN.
But none of that matters if they don't serve your destinations.

I would first figure out what your flight options are for Prague and Ljubljana. Often the price, departure time, and where I am staying drives my choice. If you use the airport code ”LON” in your flight search, you will see departures from all the various London airports. Consider where you are staying and the transit options from there.

Well it seems that Prague is served from STN. LTN and GTW (and even Southend via Amsterdam), and Ljubljana from LTN and GTW.

Thank you all for your suggestions and information. I'm staying near Paddington, so easy transport to LHR via the Express. But I'd like to try one of the other airports if convenient. Based on flight availability, I may use LHR for either the outbound or inbound flight, and try LCY or LTN for the other flight.