I'm an AA member which covers the towcar and have membership of OHTO for trailer/car cover too. AA members get a discounted price on joining OHTO, or OHTO members get a discounted price on joining AA (I can't remember which way round it was).
Have had one breakdown so far (in 10 years) where car electrics died. Called AA who came within the hour and fixed it. If they'd been unable to fix it, then OHTO would have taken horses/trailer home and AA the car. I called OHTO anyway to warn them I might need them but in the end the AA fixed the car so I was able to phone them back and say I didn't need them.
AA can tow caravans (so possibly can tow trailers?) but can't tow livestock.
We have always used Equine Rescue Services. Their equine service is second to none! They have over the years provided peace of mind both on the UK and on the continent for us.
They have a trailer assist on there which will look after you for the whole year.
OK - just to let you all know and for future reference in case someone one day searches for "horsey recovery services" - this is what I have just done.
Equine Recovery (lovely lady v helpful) told me that they will recover the pony and take it anywhere in the country for £60 a year but if I paid £180 I got 3 years - so I joined and they gave me a special code and with that code I joined the AA for a 35% reduction.
Again, FYI - I got a quote for either £7.52 /month or £74.25/year for full recovery of car and trailer to anywhere in the country or £16.35/month or £160/year you can get full recovery and they also pay the garage to fix it! Not things like brakes or wipers or things that wear out but if your alternator goes they will replace for free! Amazing. Anyway, thanks guys for the suggestions.