If you go down the converter route then you have to make sure that it resides by your hedge trimmer and the closer the better.  This is because the hedge trimmer motor requires a fairly hefty current to work. If you have a long cable between converter and trimmer then you will get a voltage drop down the cable.  The cure for that is to use a thicker cable but then that will make actually cutting your hedge a right pain in the backside.

To be honest, the best, simplest and easiest way is to get the replacement battery (I'd be tempted to buy two) and get them shipped to a friend or relative in the UK who could then ship it on to you safely packed and sealed.

These batteries are of the sealed type and I think that the UK supplier may be being over-zealous as Royal Mail say that these can be shipped under certain circumstances.

Sealed lead acid batteries, absorbed glass mat and gel cell batteries.

Batteries must comply with Special Provision 238 of the UN Recommendations on the Transport of Dangerous Goods, Model Regulations (please check with the manufacturer or distributor).

No more than one battery in any one package. Maximum weight 1.5kg. Item must be protected against short circuit (by the effective insulation of exposed terminals) and securely packaged.

Package must be marked “NOT RESTRICTED” and “SPA67 / SP238”. The sender’s name and return address must be clearly visible on the outer packaging.