I don't know about Bradcot v Isabella, but I can comment on Dorema v Isabella - and surprisingly, you may think, Dorema gets my vote...
I first owned a 2.1 deep metre Dorema, which was hugely satisfactory, but succumbed to a temptation to buy the 3 metre deep Isabella Commodore.
The Isabella may have been better canvas, but was inferior in a number of areas. At the inside corners, the canvas only went half way down the van's runners, making it a poor seal in the corner, and a seemingly unnecessary flaw. At ground level, the inside fold was flimsier and shorter than the Dorema's and left significant holes where it had to be tucked around the vertical posts. (My wife went round stapling polythene strips to it, to close the gaps - no such problem on the Dorema which has generous overlapping and seals completely.)
Then came the rains. The Isabella let water seep in through the gaps at ground level and dripped inside from the roof at the top of the front corner posts. We had to pack the holes with make-shift rubber washers. The Dorema has never let in a teaspoonful of water! The IXL poles were OK but definitely bowed a little - Dorema's steel ones are rock solid.
I have now changed my van and took the opportunity to go back to Dorema because of needing a larger size. Put it up last week and it was every bit as good as the first one - better in fact because of the standard clip-on storm straps.
So my vote goes to Dorema. I too would normally happily pay more for the best, but in this case I don't feel Isabella is the best, and certainly not to the extent of paying more than twice as much for something that, after all, is only a tent!!