Gravel bike tyres on a 30 mile Sportive

Well I thought they were pretty. 

There's no pretty much about it. They are road tyres. 

Your tyres OP, (if I've googled correctly) should be fine at higher road pressure. 

If it's a one off ride and don't expect a fast ride compared to a road bike, then enjoy it with your current tyres.

You've pretty much got a road tyre. 

Personally I think you will be fine of the tyres that you are already running.

But if you are thinking of changing them, the you could do worse than these;

http://www.panaracer.com/gravelking 

I'm running a pair of Panaracer Gravel King tyres (700x26c) on my gravel bike.  I've covered 100's of miles with them on trail, cyclepath and rough British country roads. I picked them up as a "cheap" alternative to a pair of 27c Vittoria Open Pave tyres. I also have a set as my winter tyres on my road specific winter bike.

I can't really tell the difference between the Panaracer and the Vittoria or between these and a road specific tyre, it really is a fantastic tyre. In my opinion.

Missus' road bike came equipped with Zaffiro's (25mm). Roll fine, definitely won't slow you down. These aren't really gravel tyres and like bender says above, if I was going offroad, I'd swap them out for something tougher, and just use the Zaffiros on road rides.

Zaffiro are road tyres aren't they? So absolutely fine. 

What I would say is that, for any gravel adventures, you'd benefit from some gravel tyres on the Orro such as the Schwalbe G-One. 

And, frankly, I'd leave them on for a 100 mile sportive they are that fast.

I had gravel 35mm tyres on my giant revolt 2 and changed them with 28mm slick road tyres and noticed the immediate difference with speed and distance covered in lesser times, you can always keep your gravel tyres for winter training,

I shouldn't think those tyres would slow you down unduly for a 30 mile sportive Ted. Have you ridden 30 miles with these tyres?