Is this their first visit or second?
If it is their first visit, do not let them in and they can't do anything.
This isn't entirely true. They can still lawfully Levy on items outside the household. If it is the First visit. The Bailiff will simply be looking to Levy items inside and outside the household, and not take anything. You will be asked to sign a walking possession order, allowing them legal access to enter your home a 2nd time without your permission (and use a locksmith if needed)
If you do not open the door to the first visit, they again, will simply levy on items outside, and because you have not signed the walking possession, your bailiff fee's will increase a large amount.
On Priority debt, such as council tax, child support, road tax's, your debt file may be returned to the client who sent your details to the bailiff, and you could be liable to further action, for example Deduction from earnings or even a prison sentence in some cases.
If you have already let them in on their first visit, they are permitted to enter your property - obviously not forced so lock all windows and doors
Again, if you have signed the walking possession on the first visit, they can legally use a locksmith to enter your property and remove all goods on the original Levy. Bailiffs tend to get police to be in attendance while they remove products.
They can't break into your house if you have no walking possession signed, they can only use openings that they find on your house, for example an open back door, window or sky light.
If goods have been Levied on outside, they can remove them if you refuse them in. However, if these items do not cover the amount of your debt, and bailiff fees, then you're leaving yourself open to further action