Should be no problem in either direction.Take your package to the PO, tell 'em where you want it to go, pick the service you want (tracking, insurance etc.) and fill out the customs declaration form to go on the parcel.Coming the other way, if it's not a flick/gravity/flipper/AO knife you should be fine. Owning and importing locking blades is not a legal issue in the UK. There may be customs and handling charges to pay, so be prepared for those. It will be your responsibility if the item gets confiscated though, not the seller's.
While I've heard of some OHO knives being stopped coming into the UK, that's being done on the argument that they can be flicked open with a snap of the wrist and so count as gravity knives. I've yet to see a MT that could be guilty of this.
Oh and as to what Royal Mail will and won't handle, it's actually a much shorter list than you might think and they are most certainly allowed/obliged to take sharp objects such as knives and whatnot.https://www.royalmail.com/business/system/files/Royal-Mail-Prohibitions-and-restrictions-UK-and-international-mail-contract-and-account-customers-June-2015.pdf
I say Camping Tools
Congrats. When we open up our trade and sell parameters lots of wonderful deal are to be had. I was a bit nervous and I've had my share of scary moments but all has worked out on both ends thankfully. We have some super members and some tools that reside on opposite sides of the pond. No reason we shouldn't reach across and enjoy.