how meny lumens do you think it realy is
320 maybe 
I guess a good question is what LED? It says Cree so my guess is it must be an MC-E
It is going to be a lot bigger than it looks - it says it wants 4 D cells. It should be possible to get 750 lumens off the emitter without too much trouble, but 4 D alkalines asked to push 2.8A are not going to last a long time.
And since it is £70 - around US$110 it doesn't seem all that great value for money.
My best MC-E, the Ultrafire 510B with the M bin MC-E emitter (US$20) is not far off 600 lumens which is probably what this Tesco light will do with good NiMH D cells - doubt it'll get close to that on alkalines. But 18650 cells are hardly consumer items.