Whilst it is advisory & there is no legal enforcement people will interpretet they way they wish.
I will not be surprised if Parliament backs up this lockdown with Regulations like they did in March. Expect legislation to be passed this week. Accordingly, it will be a criminal offence to leave home unless you can prove the journey is reasonable under the guidance, and somehow within the specific exceptions listed in the Regs without stretching the intention of the legislation too far. Expect fines or other penalties for those caught in breach.
How much resource will be provided to police travel on the roads is a valid question, but "I don't think there is much chance that I will be stopped" is not a good excuse to take your MH out for the day to enjoy the countryside or forest or other scenery even though it would make you feel better. I am sure we would all benefit from days out after 5th November, but it really defeats the purpose of the new lockdown.