Any way to specify £2 for 1st item, 30p additional item?

[schoolboy]

Can't even sell by weight due to the Royal Mail's ridiculously overcomplicated sizing system.

I disagree... I have set up several sites where RM is the shipper for items that can be large or small. It sometimes takes making FOUR CLONES of zonetables module (to cater for UK postal zones and differing weights for letters, large letters and packets / parcels) but you can set up your modules to handle weight - based on the data you find

HERE

and

HERE