Have taken some 'expert' advice on this now.
Mode 'S' will not appear anytime soon in UK. Although its essentially a software upgrade for the ATC radars the main UK radars require to be replaced (not only for mode 'S'). NATS had a 10 year plan to replace them all, but the cost is now £150million and with the HUGE debt NATS has the interest repayments have crippled that size of investment.
In UK during the summer usage of mode 'A' squawks reaches 95%.