Do not let them in, speak to bailiffs through the letter box or a window (upstairs one!). Once they have gained peaceful entry, they are allowed to break in on subsequent visits.
Usually the first time they enter your property, they will make a 'walking possession' order - basically they will make a list of all goods and the values. The debtor must sign this.
They will leave the goods where they are and make a repayment agreement with the debtor, if this is not stuck to they will return, possibly force entry and sieze the goods listed on the walking possession order.
Your BiL would need to prove that the goods were not his - were they bought on credit and therefore there is a credit agreement? A bank statement showing the purchase etc?
It obviously would be best to cough up what he owes before it ever gets to the stage of having goods siezed.
If you Google bailiffs, there is plenty of helpful info.