Britain's native forests are a combination of the Toxic Forest from Nausicaä and WoW's Zangarmarsh. Countless acres of hundred meter tall mushrooms with increasingly dense toxic under and overgrowth.
The native flora and fauna have high concentrations of metallic and semimetallic compounds. This leads to megafauna having semi-mechanical appearances; fur look like iridescent wires sprouting from hide, scales and shells vary from solid metallic hues to the iridescence we know from insects. Within the muscles of the megafauna are filaments of organometallic polymers that in the largest of kaiju form cables as thick as a man's arm. Within the cultivars of mushrooms with suitable mecha muscle fibers, there are similar filaments of polymers only with less metallic compounds.
Castles are large spherical or ovoid structures usually encircled by walls and fences. The oldest castles are made of metal. Troubadours sing ballads in which the castles are the ships the first king of England, Felix Brutus, sailed from ancient Troy, by way of Rome, after its fall.
All large mechanicals are owned by the lord of the local area. As a rule, every mecha is a weapon of war and fit for only the use of knights and their retinues. In practical terms, some mecha aren't suitable for war or leisure by knights. Those wholly unsuitable, which means for whatever reason incapable of hard-mounting or carrying arms, are used by the peasantry for those tasks that require the a small giant to do.
Following such unknightly mecha are those that could be used for war but incapable of bearing heavy enough armor yet still possessing other knightly attributes; these are known as palfreys and are considered suitable conveyances of ladies while on the hunt and other sport.
Next is the much larger class of knightly riding mecha which contains two sub-classes, the rouncy, the lower end of mecha a knight would take into battle, and the courser, a sturdier and faster class. Neither of which would be the first choice of any but a poor knight to take into battle, these are used mainly for sport or by squires to support their lord.
The penultimate knightly mecha are the charger class. These are purpose built and the purportedly to be the closest to how they were when Felix Brutus came to England; although, troubadours sing of larger and more powerful mecha, and many a knight has gone off into the Wastelands and Forests in search of a legendary mecha to bring home. The sumpter class has already been written on.
North beyond Hadrian's Wall are the Picts, savages that have tamed and ride megafauna.
Saxons, ancestral enemies to the Cymric people of England, are said to be half-giant, and indeed they share the giants hues of blues, greens, and pinks in their skin.
There are Things that live in the Forests and Wastelands. Things like, yet unlike either man or beast or kaiju. They work sorceries for their amusements, and sometimes those amusements involve mortals. Sometimes the mortals fare well, and sometimes they never fare at all. There are stories told of castles made of narrow spires of crystal, and knights made of solid gold, or possibly mecha; Faerie-addled peasants are poor witnesses.