If you go to the trouble of searching last years PBO's or MBM's you will find that one of them did an indepth test and something like a B&Q yacht varnish at a fraction of the cost of some came top or very close to it. As it was a feature it should be easy to spot from the cover or index. Good place to start I think, could save you lots.
Use Sikkens but first get their instruction leaflet and follow it to the letter. My Iroko toe rail varnish is in excellent condition after I was advised to do just that.
I have a teak gaff cutter in Britain and I do too!
BUT - Coelan is not a varnish, strictly speaking - and it is MILES more expensive than the most expensive varnish.
Iroko is not very difficult to varnish; it is a bit oily but not as oily as teak.
It does need to have the grain filled, so if using less than say 6 coats use a grain filler. Personally I think iroko looks too brown to really look good so I put a teak or mahagony stain on it first.
My personal preference among the ordinary varnishes is Epifanes.
I tried B&Q yacht varnish once, under the mistaken belief that it was what it said on the tin. Three coats had completely worn away in some areas after 3-4 months. In general, the more expensive it is, the better it is (tho' it's not a linear relationship). I use Epiphanes now.
I use "International Crystal for Teak" on the cabin table, as you can stand a boiling kettle on it, but I would not use a 2 pot poly anywhere externally. Otherwise, Epifanes, but for bits that really get banged about, which ordinary varnish would never stick on, like blocks, I am now converted to Coelan, which is not really a varnish at all, but is very tough stuff. It is on the linear scale, costing 4x Epifanes and seems to last 4 x as long.
Epifanes make two excellent varnishes for Teak and Iroko.
Their PP Varnish Extra is two component and must be over coated for exterior use with 2 coats of either their one component Clear Varnish or their 2 pack Polyurethane Clear.
A cheaper alternative is their one component Wood Finish Gloss.
They publish an excellent Systems and Advice Booklet.