Stavanger and Bergen are both good entry points as they both have flights from LHR, but Oslo also works great for Norway in a Nutshell.
If you arrive in Oslo it will only take 5 hours train to Myrdal, not much more than from Bergen. Arrive at OSL in the morning, take the 1201 (or 1506) train to Bergen and you're in Flaam 1810 (or 2145). Spend the night in Flaam, travel to Bergen and then to Sognefjorden / Geirangerforden. Fjord Tours can arrange everything including hotels, the tours are very flexible.
If you have time left take a bus/speedboat to Stavanger and see Preikestolen.