Is the brand shaped like an 'M'? If so, she is likely a New Forest pony, labelled for meat.
There are at least, 10,000 horse/ponies from the UK, which are exported to France and other countries on the continent each year, for meat. Some believe it could be five times that number. Passport faking is well known and many unpassported horses get through.
Strangely enough, during the recent horse meat scandal in the UK, it was thought that the meat used, had been from horses sent to Europe from the UK for meat, then used in food and said to be beef and sold back to the UK in packaged products.
It would seem odd that your mare, would have been imported back into the UK. The tag and brand do suggest she was headed for meat though. Maybe someone spirited her off the trucks or she escaped somehow and ended up with you. Did you get a passport on or with her?
Send me a pic of her tag and brand and I will likely be able to tell you a lot more about her.