As above Simon is your best bet - I'm up at SRR on the 16th for a remap sure he'll still have spaces.
Not sure it will be an STI lump as the UK300 was a special edition before the STi Bug was released in the UK:
The UK300 was a joint project between engineers at Subaru (UK) Limited and Prodrive Limited of Banbury, with major styling work carried out by Peter Stevens who designed the famous McLaren F1 and Subaru WRC rally car. As it name suggests it was uniquely available in the UK and limited to 300 units.
Key changes include new headlamps featuring black gloss shells with twin high-intensity Hella light units identical to the Impreza rally car. Sitting above the twin lights are small diameter circular indicators. The new Peter Stevens designed front and rear aerodynamic aids reduce high-speed lift, enhancing tyre grip and handling control. For example, at 100 mph, front lift is reduced from 35.6 lbs to 27.8 lbs while rear lift goes down from 29.6 lbs to 20.7 lbs - a reduction of 25.6 per cent. Only one colour has been specified for the UK300 - WR Blue Mica - which is also extended to the WRX's standard deep side skirts
The UK300 also boasts gold-coloured OZ 10-spoke alloy wheels measuring 18 ins by 7.5 ins and carrying the latest 225/40 Pirelli P-Zero Rosso tyres - up from the standard WRX's already generous 17 x 7 ins alloys with 215/45 tyres. Other exterior enhancements include white side indicators and white front markers.
Inside, the UK300 features blue suede-effect seats - the front being the rally-style bucket type - plus matching door inserts. The potential collector's car also has special deep-pile carpet and boot mats plus a limited edition numbered plaque.
Not only the exterior and interior are upgraded but the engine was fitted with the Prodrive WR Sport package. The secret to the £1,600 Prodrive WR Sport performance pack option is a reprogrammed Electronic Control Unit (ECU), uprated intercooler piping and free-flow exhaust system with a large single tailpipe. Both power and torque have benefitted from the package with a power increase from 218 bhp to 245.
Looking at your pics I'd say little of the UK300 remains due to the full Morrette body kit and lights.
UK300:
Morrette Kit
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2005 Spec B 6MT - KiDO tuned (~270Bhp)
Custom exhaust - Cosworth filter - Samco Intake - Tints - Bilstein B8/B6 & STI springs - SuperPro ALK - Whiteline positive shift kit, rack mounts, rear adjustable camber bushes, front ARB mounts & strut brace, 20mm rear ARB with AVO mounts, - Hardrace Gearbox and pitch mounts - Prodrive PFF7's - Front Brembos - Kenwood CarPlay - 1/2 carbon grille - STI front splitter