In the UK, like many other countries, we operate a semi-circular rule for altitudes. This applies to both inbound and outbound aircraft.
Essentially, the rule is simple;
Aircraft flying East will do so on an odd flight level.
Aircraft flying West will do so on an even flight level.
When talking about even or odd flight levels, we refer to the number second-to-last in the flight level (.e.g FL190, FL240, FL60)
This rule relates to the magnetic heading that the airway is going to take the aircraft, however for the most part usually it is correct to assume the geographical location of the destination aerodrome vs the geographical location of the origin aerodrome (whether the destination aerodrome is East or West of the origin).
While trivial, this rule ensures the safety of thousands of aircraft every day, by ensuring at least 1000ft of separation between aircraft travelling in different directions. Essentially, it avoids head-on collisions between aircraft.
As a controller, you must check that the aircraft is flying the correct flight level according to this rule.
Note: Some countries operate a North/South semi-circular rule, such as France, Italy and Spain. As such, there will be times where aircraft filing an odd Flight Level when flying South, even though the airport is in the West, may still be correct. In these cases, it is best to use your own judgment and decide if the level the aircraft has filed is correct or not.