Rapalas are good .
I got this one in 1973 .
Still use it .
I have a Marttiini that also says Rapala on the sheath.
Though not used much anymore due to lack of opportunity to go fishing should the need arise the handy 40+ yr old Rapala still does the job.
I have a schrade like this:
http://knifeextras.com/products/438900?utm_medium=cpc&utm_source=googlepla&variant=10970315972&gclid=CPf1sfuf19ACFUWRfgodKwYKzg
here as well:
https://www.knifecenter.com/item/SCH167UH/schrade-167uh-uncle-henry-steelhead-fillet-knife-blade-delrin-stag-handles
it has served me pretty well over the years, i bought a spare awhile back and generally use it in the house.
I got on a knife kick awhile back and found a few global knifes on sale that are really nice. The price is typically nice though, so i wouldn’t have gotten it had it not been on sale (deeply discounted). This one holds a edge very well and is used for the Thanksgiving and Christmas turkey:
https://www.knifecenter.com/item/GLG21/Global-G-Series-G-21-Kitchen-625-inch-Boning-Knife-Flexible
Doubt you want to spend that much, but they sure are nice and as they say, you only live once, right? 
For filleting flounder, i typically will use a “sharpfinger type knife and run from the backbone to the fins with something like this:
https://www.knifecenter.com/kc_new/store_store.html?usrsearch=sharpfinger&
Helps to get all the meat off the bones better than a flexible knife will.
Good luck, hope you catch a bunch to get to use them. I haven’t had much time to fish recently, but i keep saying i will next year…
https://www.bubbablade.com/collections/knives/products/bubba-blade-7-inch-tapered-flex-fillet-knife
I’m a big fan of my Bubba Blade. I have the larger flex blade for filleting saltwater fish.
Victorinox 40710 is a real nice one and not too expensive
I have to second the suggestions for the Dexter Russell, Rapala, and Victorinox (with Fibrox handle). I have used all three, and honestly, they really are great values for the money. They hold their edge fairly well, but are easy to sharpen, and have held up through years of filleting salmon, trout, striper and anything else I had.
I know victorinox and Rapalas are available in the UK, so I would suggest checking online through some of the UK tackle shops. I know there are plenty, I’ve ordered from some of them in the past.
Here’s a link to Amazon UK for a simple victorinox fillet knife: Fillet knife
I can’t say enough about the fibrox handles. They really are great - super nonslip even when wet. They hold up well too.
I have a henkels, and it just sits in the box. It’s sharp and pretty and cuts well. But, it’s heavy in the hand so it’s consistently overlooked.
I’d have to agree with keltex78 about the Bubba Blades!
I personally use a Bubba Blade. The best filet knife I ever bought!
Well worth the price! I purchased this one: https://www.bubbablade.com/collections/knives/products/bubba-blade-9-inch-tapered-blade-flex-fillet-knife
Cheers,
Willie
Darn, sorry dude.
Some really nice knives linked to, thanks very much for the suggestions.
It seems that knives are much dearer here in the UK, than for example the US, & the quality of knives from our fishing tackle shops is much poorer.
If that Dexter had been a reasonable price over here I probably would have bought one.
I have not been able to find any UK supplier with the Martiini 6” economy” knife I already have & TBH fancied getting another make of knife to try.
I may buy a knife from outside the UK as even with shipping it will only be around the same price as one of our fishing store knives but much better quality.
For example I have found a Dexter P94813 for $11.85 on ebay US plus $16.57 international shipping so I assume that it will ship to the UK.
That is about £24 total so £38 or about $50 cheaper than buying the same knife from Amazon UK.
Thanks again for all the suggestions.
Not perfectly on topic (fish filleting knife) but I found it so interesting that it changed my view. I happened to watch several butchers (slaughterers) from different companies at work, cutting whole pigs and cows into pieces. They all used knifes with cheap-looking blue or yellow plastic handles.
I asked one and he said they are not using fancy bolted wooden handles like chefs do because they are not as hygienic and the grip is not better than with the ergonomic plastic handles. Better chef knifes are forged instead of punched but with the amount of wear the industrial knifes face (re-sharpening every few hours) it would be too expensive to buy nicer knifes.
However he said cutting quality and durability are the same. They would not accept anything else.
So if you are looking for a real tool for the job without any bling take a look at industrial knifes for example by F. Dick (blue) or Marsvogel (yellow, black).
WAF (wife acceptance factor) and your own chef feelings in the kitchen may need to be worked on though.
Few here for £15 or so upwards plus a bit of postage (free on orders over £30)
https://www.heinnie.com/catalogsearch/result/index/?cat=121&q=fillet
SP
Thanks for the link Paul, I always forget about about Heinnie Haynes here in the UK, don”t know why :person_facepalming: