Wrong. Plaxton was sold off, and the remaining companies were moved into Alexander Dennis, as stated in a comment I made before, below:
You say that, but their independence only lasted a couple of years - they were very quickly bought out by ADL (soon after launching the Centro and Primo). Their own bus range was soon discontinued in favour of ADL's Enviro range and they are now the dedicated coach-building arm of the company, though I believe their Scarborough factory also assembles a fair few Enviros on the side.
The E200 and E400 were both new designs introduced two years after the formation of ADL, so I'm not sure where your claims that they never changed Transbus designs are coming from. Yes the chassis was (initally) unchanged from the Trident, but the body and dash were all new.
I'd also like to point out that one of the "Chaps"/"Three Scottish Businessmen" who bought the remains of Transbus was called Ann
Brian Souter's sibling Ann Gloag, co founder of SC though she now has little to do with the company and spends most of her time on charitable work. The pair of them hold a 55% stake between them in ADL, leading to the common misconception that SC/Brian Souter owns ADL.
As to the original question, the Transbus E300 was a lightweight, full-size, integral bodied single decker featuring the same (imo quite handsome) revised body design as the original Enviro 500. The ADL E300 was both a fully integral bus which directly replaced - and was largely based on - the Transbus E300 (though with the new 2006 Enviro-family front end design), and a heavywight single decker body produced for the Scania K230UB, MAN A69/18.240 and (rarely) Volvo B7RLE chassis. Both were ditched with the introduction of the MMC series as heavyweight single deckers have largely fallen out of favour in the UK, and with the introduction of a longer version of the Enviro 200 the integral version had become redundant as a separate product.