on a post 97 liscence you can drive a car of 3,500kg MAM towing a trailer upto 750kg MAM

OR

you can tow a trailer with a MAM greater than 750kg so long as the combined train MAM does not exceed 3,500kg.

even if you are within these limits you can NOT exceed the towing capacity of the tow vehicle!!!!!!!!!! so you can't tow a focus on a trailer with a micra to keep within the licence weight limits.

to exceed the above you need to take a towing test and have the entiltement added on your licence.

thats the easy bit.

now you need to know what car your going to tow, on what SUITIBLE trailer and get the MAM figure of the trailer plate. if this exceeds the towing capacity of the focus you intend to tow with (even if you don't load the trailer to its MAM) then its illegal even if it falls within the 3,500kg train MAM rule which i suspect it won't!

and all this is before you've checked the trailer is in good working order, including the lights, fitted with the number plate of the tow vehicle and hooked up.