I have brewed one of these Wherry kits, and beyond any question the beer they produce is the closest any homebrew can ever come to a real ale as served in a good pub! I did not even use a hydrometer - just a close fitting lid which the escaping Co2 could push up enough. Once the gas stops rising, you bottle the beer, and if you miss by a day or so, no big deal as the Co2 is heavier than any air that might find a way in to oxidise your precious brew.
You cannot go far wrong, and you should be in for one of the best pints you ever brewed!! Just make sure everything is spotlessly clean and sanitised.