Towing trailer with a fiesta.

[fastcaterham]

Having lost use of the van I used to use to transport my kart around I'm now looking to tow it on a small trailer (combined weight ~150 kilos)using my mk5 fiesta 1.25 zetec. Having looked at the manual though it gives a maximum towing weight of only 250 kilos braked and no weight at all for unbraked. What exactly are the reasons for this and is there anything I can do about it or is towing with a fiesta basically a no no?