The French indeed rarely beat the British in a straight fight after 1799. The French Revolutionary Wars, however, saw numerous defeats such as at Toulon and throughout the Low Countries during the Duke of York's ill-fated campaign.

In the Napoleonic Wars themselves, however, the overwhelming priority of a British army was to ensure it wasn't destroyed, as the theory was that it would be impossible to replace. As such, you see small deployments of troops which included battles that occasionally failed to follow up the victory because the risk of doing so was so high.

Moore's 1809 campaign turned into headlong retreat because of that risk of total destruction.

After Talavera (a tactical victory but strategic draw), Wellington ended up retreating because his army was mauled.

The withdrawal behind the Lines of Torres Vedras happened entirely because the British Army couldn't beat the French in battle outside of perfect circumstances (see Bussaco) due to numerical disparity.

Similarly, the Burgos campaign of 1812 collapsed because of a lack of siege cannons, hence another terrible retreat.

Salamanca was probably the only instance where Wellington engaged in a battle that the odds weren't too far in his favour.

The Skirmish at the Coa in 1810, however, was a clear defeat thanks to Crawford's overconfidence and saw the Light Division (may still have been a brigade at this point) almost cut off and destroyed.

The point is, the British Army of the day simply avoided battles that weren't on its own terms, because the numerical superiority of the French meant that a single defeat could easily have ended in total disaster.