I don't know about any UK suppliers, sorry.
The kits that have oak in them usually are balanced pretty well. Of course, you can add more, but as you mentioned overdoing is possible. They have oak spirals available, which is a "slower release" type of oak, and oak chips both French and American (and I think Hungarian is common). You can actually keep tasting the wine until it's almost too much and then rack it off the oak. Then, it'll age into a perfect amount. If you overdo it, though, it might take years to age out.
I don't know if four sachets are too much or not- I guess it depends on the depth of the oak flavor and the amount of the packages. Oak sawdust gives up its flavor faster than the spiral or chips, since there is more surface area exposed to the wine. I'd start small, and add more if it needed it. You can always add more, but you can't remove it once it's in there. If you're a patient person, I think the spirals or chips would be the way to go, along with tasting once a week to see where you're at in the "oakiness".
I'm making a kit now that had one package of oak sawdust in primary, and 2 american oak spirals for secondary. It's meant to be on the oak for 40 days.