This was the problem, also the larger SR.177 was built in the same philosophy.
Perhaps an RR Avon Mk200 series could be replace the Viper/DH Spectre combination, the SR.53 would have then 9,500 lbf of thrust. SR.177 needs at least 24,000lbf, choice is more difficult. And of course this mean more money into the concept.
Quickly, two links with several proposals. In one link the CF-105 Arrow is proposed, but here my motto is "Only British Aircraft", the Arrow is Canadian, so is rejected.
http://www.rp-one.net/f_155_t/fd_iii.html
http://www.rp-one.net/f_155_t/f155t.html
Another link, for what if Fairey Delta III:
http://www.whatifmodelers.com/index.php?topic=12289.0
P.S Of course, if something is good, my AU can use it replacing other types of aircraft.