The old saying about getting blood out of a stone is worth remembering here. Some people on benefits might (because they've only recently fallen on hard times) own some expensive items which it would be worthwhile bailiffs seizing but most won't. Also no court is going to issue an enforcement order which would leave the debtor with insufficient money to live on.
If you've not already done so, the first stage in pursuing a debt is to send a formal letter, demanding payment. (That letter has to include your name and address, the debtor's name and address, the date, the amount demanded, the reason for that demand and a statement that, unless payment is forthcoming within a specified period, legal action will be commenced).
If you've done that, but not got your money, the next stage is to initiate court action through the online process here:
https://www.moneyclaim.gov.uk/web/mcol/welcome
You then have to wait for the court to make an order in your favour. (Remember, however, the court will only order what the debtor can actually afford. With someone on benefits, that might only be a few pounds per week).
If the debtor then keeps up the repayments, there's nothing further that you can do.
If the debtor defaults, you have to return to court to seek an enforcement order. (You can do it via Money Claim Online if you want to send in bailiffs). The options available to you are listed here:
https://www.gov.uk/make-court-claim-for-money/enforce-a-judgment
Remember that sending in bailiffs often doesn't produce a lot of money to pay off a debt. For example, the debtor might have a laptop which he paid £350 for. When it's sold at auction, bidders won't know whether it's fully working or not, so it might only be sold for £60. The auctioneer's commission might be 15%, leaving just £51, but then there's the bailiff's fee to come out of that as well. So probably you'd only get a few pounds from the sale of his laptop.