If it is more than 6 years since you last did anything which could be interpreted as acknowledging the debt (such as making a payment, or agreeing to make one, or agreeing that you owe the money) then legally they can't do anything unless they have already got a ccj within the 6 year period.
They cannot do the first 2 things they are threatening you with without first getting a ccj and you defaulting on the terms of it. They can't do the third thing (make you bankrupt) without first issuing a statutory demand to you. This is very unusual because it costs them a lot of money to make you bankrupt. Their letter is probably a bullying threatening bluff.
You need to get independent free advice - CAB, or CCCS. Also, try to find out as much as you can about when the debt was incurred and when you last had contact about it.