Don't know about the UK, but in En Zed when the aerodrome controller is unable to provide reduced separation (due cloud/viz/whatever) we use a table of distances under delegation from the approach unit. The distances are tailored to a particular performance group, so fast on final vs slower to depart, the distance is considerably larger than the vice versa. Typically at my unit the distances range from 4nm up to 8nm, or more for a particularly low performance departure. This is supposed to guarantee radar separation in the event the arriving aircraft makes a missed approach.
When the weather is above IFR circuit minimas for the category of aircraft, and the aerodrome controller is able to provide visual separation, everything speeds up considerably.
The procedures are aerodrome-specific.
In your situation, with a departure airborne and an arrival on 2 mile final, you're probably looking at about 3 miles between them. I don't have any idea what separation is being applied, and it would seem you don't, either.
Your unit standards officer or senior controller, or the parent approach unit should be able to answer questions around this.
One would hope.