I don't fish on charter boats but from what I know about boat fishing and the charter boats I see out there and such I'd say it's a bit of a lottery as to whether you get a good day or not regardless of tides , it's not just the size of the tide you have to take into account but what time of day high or low tide is etc. I think you just have to look in your diary , find a date you can go , check with the skipper that he's available , book it and trust in the gods that sea, wind and tides are favourable , I can go at the drop of a hat and still find it difficult to "pick" the perfect combination , more often than not the dodgy looking days turn out better than the ones that look spot on in the forcasts.
When does the fishing start to get good? You can't beat the spring run , look up the catch reports around April and see the numbers of fish being caught , the charter skippers know where these spring fish hang out and always have good catches on them.
Duncans "slow" boat is a bargain at £30 for the whole boat, I'd take him up on it if I were you

I reckon you can do the sums , a boat holds 12 anglers and charges £30 a head which is £360 , so unless the skipper is willing to do a discount for a private charter (which I have no idea if they do) that is what it's going to cost , for a slow boat
Have a look at the spring run catch reports , pick a date in the middle of the best of them and book up

(and hope for the weather

)