Zdas04, as usual, offers excellent advice here. I confess I don't know what GMISS is and mr google didn't seem to know either, but I'll assume it is some sort of integrity system.
Don't forget B 31.3 offers you up to 33% over the design pressure for less than 10 hours per year. (302.2.4 f)1)a) ) so your comment that "this shall be the new design pressure " is not necessarily true depending on the duration and likelihood of the event.
I don't know any other means of re-rating pipework like this other than a hydro test, but even that depends on what "spare" you have in your wall thickness to up your design pressure, but as noted above, it doesn't have to be to the full 18 bar, just 3/4 of it if this event is very rare.
Basically you need to look at how to alleviate the surge pressure before anything else and make sure it is properly analysed on a transient model, but it seems quite high to me. Surge tanks, relief valves, accumulators etc can all help to get this down to less than the 13.3 bar allowed.
A few more details might help - size of line, wall thickness, flow velocity, temperature, fluid, length, that sort of thing.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way