in the file includes/languages/english.php you will find

define('TEXT_PRODUCT_WEIGHT_UNIT','lbs');

and

define('TEXT_SHIPPING_WEIGHT','lbs');

chaning those will list your items in Kilos,

then you should go to configuration > shipping/ packaging and look at the things with Tare in the rows, it;s the amount you add to each pakage to compensate for the increased weight due to the box or envelope. I normally recommend 10%:0 for the smaller items and 8%:0 for the larger ones, you'll need to take some items yourself just to work it out but generally that works, otherwise you'll find that zen cart adds 3 Kg to your packing for each item which creates ratehr a jolly mess.
https://www.zen-cart.com/tutorials/index.php?article=85

Also after you've tested everything, the current recommendation is to to set "Hide Invalid Shipping" in each of your shipping modules to "True", they are set false by default so you can see your shipping modules are working like Airmail when you are using a UK estimate to test, but this morning a fault was reported with Paypal Website Pro (putting things in bold so people will bother reading them) where if the friendly messages like "Airmail does not ship to your country" are on, then Paypal charges £0.00 for the shipping as it goes for the lowest cost option, even if that option is invalid.

Thank you
Philip.