In 20 years, I've flow into FRA 7 times and out 6 times, but only one time (one of the six departures) included a transfer, but most of my experience was in the first 10 of those 20 years. Hopefully someone with more recent experience can expand on my advice.
If you go to the Frankfurt Airport website transfer page, (click on the red banner), you will find that all arrivals and departures are currently taking place in Terminal 1, Piers (their name for concourse) A, B and Z.
Piers A and Z are actually two levels of the same building. The upper level, Pier Z, is for passengers who are still outside of Schengen. The lower level, Pier A, is for in-Schengen arrivals and departures. To go between the levels, you have to go through passport control. Each gate is shared between the two levels. If you come into/go out via a gate, you are channeled via stair from/to Piers A or Z depending on where your flight is coming from/going to. So if you come in from a non-Schengen country and go on to a non-Schengen country, you go through, but stay in, Pier Z and never have to go through passport. It's only if you go out of the airport, or to Pier A for an in-Schengen flight that you have to go through passport as you leave Pier Z.
Pier B is similar, except it is all on one level, but is partitioned into in-Schengen and out-or-Schengen (transit zone), and you go through passport control entering or leaving the transit zone.
Looking at one day's schedule shows Lufthansa flights from Varadero arriving at Pier B and Lufthansa flights departing for London departing from Pier B. That was just for one particular day; if it holds true for your travel day, and if you only have carryon luggage or if Condor allows you to check your luggage through to London, you can just stay in the transit zone in Pier B.
If, on your travel day, you have to go from Pier A/Z to Pier B (or visa versa) you should be able to use the SkyLine (little cars on a track on top of the terminal) between transit zones. (This is the only thing I am unsure of, but that seems to be what the website is saying). In 2002, I arrived at the old Pier A on a flight from Munich and transferred to Pier B for a flight to the USA. I used the tunnel between Piers A and B, which is for in-Schengen passenger, then had to go through passport control to get into the transit zone in Pier B.
As Paul suggests, talk with Condor as to whether they can check your luggage through to London. If so, you won't be limited to carryon only.