I know it is not precisely what you are looking for but there is a company in the states that is selling some T2115C step wedges for $15.20 each, one of them could be cut into three...
The T2115C is a 21 step .15 increment strip 5" x 1/2" wide with a Dmax of 3.05
The TP120-21 is a 21 step .15 increment, 2-1/4" x 2-1/4" with a Dmax of 3.05
It would be a bit of messing about as there is not so much room for the first step due to branding but if it is being used for enlarging to 8x10 I think it will be usable.
If you are interested do a search for "Exposure Calculator For Screen Printing" on either ebay.co.uk or ebay.com
As regards shipping to the UK that's $13.78 (they said they will refund the difference after sale to me) for standard mail 8-12 days but you will have to contact them first to confirm.