Trademark Trolls - anyone have any experience of dealing with them

I have been working with a US based brand for over 4 years selling their products in the UK and Europe. The brand name is TM’s in the US but we never (and should have) registered the TM in the UK. Just before xmas I received a communication from someone that had registered the brand name as a word mark - summary was telling me they owned it and I could buy it off them or else! No mention of we have products to sell and want to sell them on Amazon…nearly every response i get re-iterates we can solve this by buying the TM off them…I checked the registration and the do own it and cover pretty much every class i sell the products under…The brand have their US based attorneys looking at it… I did some research and it does look like there may be a way to get the TM cancelled if it has been registered with no intention to use and with the purpose to sell…Any advice or feedback on similar experience would be helpful