Refer to figure.
REAL LIFE ANSWER

Your aerodrome is well within area "A", the weather region looks like it will stay roughly the same for the next few hours, you have plenty of alternates around, etc., so you look outside and it's good currently, it's best to just go flying now, before the trough or occlusion get closer.

FULL EXPLANATION

The met office form 215 (F215) is a useful synoptic chart showing the fronts and other useful weather phenomena passing over the UK (F415 shows Europe also), as well as the zones of similar weather (A, B, C, etc.) and the expected weather within those zones (which is written on the right of the pictorial chart, not shown here).

The arrows on the chart show the direction and speed of movement of the weather regions and fronts for better ability to interpolate the data between different chart validity times. This chart is valid for 1800z, which is 1.5 hours in the future (it is currently 1630z).

We can see that there are 2 potential points of interest that might make a training flight unsuitable, some time in the future. Most notably the region "B", including an occluded front and a band of different weather from region "A". Occluded fronts bring low clouds, and often showers, turbulence, precipitation, low visibility, etc., unsuitable for circuit training. This region is forecast to move closer to the aerodrome at a rate of 20 kt, but given it is approximately over 100 NM away currently, this is unlikely to hit for 5 hours (100 NM / 20 vkt) after 1800z, so some time after 2300z.

The other feature is the trough to the South-East, over the English Channel, which is moving "slowly", i.e. less than 5 kt, and we do not know the direction, so it is very unlikely to cause an issue that day at least. Troughs very often cause poor weather and should be avoided for this kind of training flight.

We do not know the weather conditions in area A, however given that they are suitable at 1630z by looking outside, and there is no expected change to the weather region in the 1.5 hours leading up to the chart (given the trend arrows), then we must say that region "A" is suitable for flying for the next few hours. The flight can therefore take place immediately, and there is no need to delay it or cancel it. In fact, any delay of 4-5 hours or more would be a bad idea, as the occluded front or trough may approach and cause deteriorating weather conditions (and it will have gone dark also, but perhaps night flying is okay for this flight also, we don't know).