The rule is you must not use the horse box " for hire or reward". Your own horses are not a problem. However, this means not only can you not charge your friends to transport their horses, you can accept no reward of any kind eg £20 for the diesel, bottle of wine as a thank you etc. As long as you do it totally for free, its not a problem.
Next problem. Is your 'girl groom' employed by you i.e. do you pay her? If so, she might be considered a professional driver which means she would have to pass the DriverCPC which comes into effect September this year. This involves taking 5 7hour courses to get her DriverCPC. As you are moving your own goods (horses), not for hire or reward, she might not come under the scope of the DriverCPC scheme. Please phone the number on the last page of the link in SeaJayPea's post to check.