I am in the U.S. so no help with legitimate/ethical rescues in the UK.
What to expect covers anything from “expect the unexpected and be prepared to care for and pay for any residual (sometimes those can be long term) issues as a result of whatever abuse or neglect the horse suffered.
For example, years ago I got a seven year old Arab out of a very neglectful situation. The people had no business owing horses. The Arab was about a mid-3 on the Henneke Body Scoring System for horses.
It took months for him to gain enough weight to become rideable. That was when I discovered he had vertebra issues that would render him not rideable for anyone over 100 pounds. He was a sweet gentle soul and became a walk/trot lesson horse for small children. He saw chiropractors the rest of his life, as needed. He was also always a hardkeeper which is not a common Arab trait.
I laid him to rest when he was 29. He is buried on the farm I currently live on.
My point is, whenever a rescue is involved we never know what underlying issues will show up later. Rescues are not a way to get a horse free or cheap. While you say your companion is “very experienced”, those words are by whose definition.
I have been on a horse 72 of my 74 years and have been paying for my own since I was 12. My qualifying markers for the term “very experienced” are likely not the same as yours
Without more details, as to your companion‘s years of experience and type of horse handling experience, my suggestion is to look at well broke horses — something with a forgiving personality that you could enjoy and have positive learning experiences