First of all...dont worry. As a former boss...the law tends to favour the employee ( years of abuse caused this). Regarding unions, does the company you work for regonise one, if you work in a shop ( usdaw) . If not Acas is the first port of call. First, try and arrange a meeting alone with your boss, have prepared questions on what is bothering you and jot down the answers ( stay calm and dont mention unions at this stage). Most bosses in the UK are poorly trained and they tend to believe that because they are the boss they are more intelligent then you. Be smart, be honest and and hopefully you can come to an agreement. Suggest that holidays or gap years are submitted in writing. If this does not work then suggest you will be getting employment advice from a specialist and would they be prepared to answer to them.
As I mentioned earlier...small shop owners have little or no training and the local CAB can also help
good luck