Hey,
I'll answer your second question first. There is nothing realy you can prepare for the initial online test. Just make sure you are rested and calm, you'll get to practice the test before taking the actual test. Secondly, the NATS application process is not a competition, as long as your scores are above what they demand you'll keep on going.
The answer to your second question is no.
As a UK citizen you can besides NATS as well apply via private companies, I think this will require you to pay for your training yourself. I don't know anything about that myself.
Besides these two options there are as well other options. The question there is just, what would you like to do and are you willing to move outside of the UK?
You could apply to Eurocontrol, requires you to move to Maastricht in the end and you will only be able to do Upper Airspace.
Besides this you can in principle apply to all countries in Europe. Some countries have different requirements than others, some will ask for a certain higher degree, some will require you to be able to talk a different language besides English as well....
All depends on what you want and what you are willing to do for it....
Good luck with whatever path you choose...