New bike for commuting and touring
Last year I bought a steel tourer.  It seems to tick all your boxes or near enough: £900, 27 speed, pretty rugged (timelapse of a ride I did last year, starting when I hit the gravel, before it got rough), and I can also do 300km days on road. I weigh less than you but then more than make up the difference on some trips.

A tourer's not your only option.  Cyclocross bikes and adventure road bikes (the 3 categories overlap anyway) will also do it but I wanted to be able to carry a lot, and you might do too.  You may want to swap tyres between rough gravelly stuff and serious distance. The combination of a heavy rider, potentially crashing through potholes (though I could be maligning you there) and a light road bike isn't a good one, which is a reason to go for a sturdier frame.

I'd steer clear of Halfords both because they don't have much range and because I've been unimpressed with their ability to sell bikes that actually fit the rider -- fit is important here as you want long rides, and drop bars make for a less forgiving fit IMO.

Mine came from Evans, but an independent local bike shop can often get similar pricing, and would have more chance of tweaking the spec if necessary.  When it comes to tyres, I suggest going for tough touring tyres, with a Kevlar belt. I run that sort of thing on my top 2 bikes by mileage, and I don't think I've had a puncture this year (over 8000km).  

As you appear to be in the UK, do you have access to a bike-to-work scheme? Is that where your £1000 comes from?