You could try the GRK as Tim mentions, these are brilliant screws, these are best purchased via Silva Timber with the nearest place to you being in Uxbridge.
The alternatives would be,
No7 Finishing screw 63 x 4.8mm by Carpenters Mate, 316 stainless steel square drive trim head £197.20 plus vat for 2000 screws
Durafix screw 4.5 x 70mm by Millboard, 304 stainless steel Torqs 20 trim head £35.00 plus vat and delivery for 350 screws
You could ask Simpson Strong-Tie, I have no experience with these, ask for Swaneze trim head or Woodpecker Trim Head.
Let me know how you get on Gary