Go to
http://www.lambdasensor.com/main/subaru3.htm
This will take you to OE fit sensors showing different connector styles.
These cost a bit more but are direct fit as long as you get the correct connector and cable length.
Or select the one you think you need and then go for the universal option, which are half the price.
"Just Lambda part number 250-23000
Description - Universal Zirconia 3 wire
£36.00 inc VAT & UK Carriage"
Or Bosch equiv but universal connector
"Part number LS02
Description - Bosch Universal
£50.40 inc VAT & UK Carriage"
the one you linked too looks the wrong size.