Coincidentally, I am currently devising a recipe for my next beer, which will be an English bitter. Looking at recipes, many of them are bittered with Goldings, sometimes with the addition of Fuggles. Goldings tend to be more common for finishing than Fuggles, although a couple of recipes I've seen in
Brew Your Own Real Ale at Home, which is a book put out by CAMRA (Campaign for Real Ale), do use Fuggles late in small amounts.
Have a look at this thread on bitterness of tins of Coopers concentrate:
viewtopic.php?f=2&t=9932&start=20#p99360I would be worried that if you start with a Coopers English Bitter can that you're going to end up with a beer that's way too bitter after you've added your other hops.
Coopers Lager is always a good base. But the Draught looks like it's only a little higher in bitterness (you might want to check on the Coopers site whether this has changed). I'd aim for 35-40 IBUs or so in total.
If you haven't done so already, get yourself some brewing software so that you can work out how much the extra hops will contribute to bitterness. Brewmate is a popular free option, while if you want the Rolls-Royce fork out the $30 or so for BeerSmith. I reckon the late hops may be enough on their own to get you to target IBUs, without any bittering hops.
The recipes in
Brew Your Own Real Ale at Home don't seem to be too heavy-handed in late hopping. 7-15g total in the last 15 mins seems to be about all they recommend. Having said that, it's only one book that I've been reading so I am sure there are other perspectives!
Cheers,
Oliver