BAE146 CREW LNDG STL ARE VECTORED TO FOLLOW TFC ON FINAL THAT RESULTS IN A MISSED APCH BECAUSE OF SPACING.

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STL ARPT APPROX 600 FT MSL. A GOOD ROUND NUMBER FOR ALT BASED ON DISTANCE FROM THE FIELD; 300 FT PER NM TO ALLOW FOR A NORMAL MANEUVER TO LAND. THERE WERE SIMULTANEOUS RWY 30L LDA/DME AND RWY 30R ILS APCHS IN USE. WE WERE BEING VECTORED TO JOIN THE RWY 30L LDA/DME LOC IN VFR CONDITIONS AT 4500 FT MSL ABOUT 10-12 MI OUT (TYPICALLY; STL APCH WOULD HAVE US AT 3500 FT MSL BY THIS POINT). WHILE ON A HDG TO JOIN THE LOC; WE WERE TURNED AWAY FROM THE INTERCEPT (MORE PERPENDICULAR TO THE COURSE); BUT NOT SPECIFICALLY TOLD NOT TO JOIN THE LOC. AS THE INTERCEPT CAME IN; WE CLARIFIED AND CONTINUED ON THE ASSIGNED HDG. WE WERE ALMOST IMMEDIATELY TURNED BACK TO JOIN THE LOC FROM THE OTHER SIDE. WHILE IN THIS TURN; APCH GAVE US A DISCRETIONARY DSCNT TO 2100 FT MSL; AND ASKED US IF WE HAD TFC AT OUR 12 O'CLOCK POS; 2 1/2 MI; 500 FT BELOW US. WE DID NOT HAVE THEM; BUT WE DID HAVE A TARGET ON OUR TCASII LESS THAN 2 MI AND 500-600 FT BELOW US WHICH WAS GIVING US A TA. IF WE HAD BEGUN OUR DISCRETIONARY DSCNT; IT WOULD HAVE BEEN AN RA. ONCE ON THE LDA LOC; WE WERE TOLD TO FOLLOW TFC FOR THE VISUAL. THIS TFC WAS APPROX 2 MI FROM US LINED UP WITH THE RWY; WE WERE ON THE LDA LOC AND THUS 2 MI APART; BUT NOT 2 MI IN TRAIL OF AN ACFT WE HAD NEVER ACKNOWLEDGED HAVING INSIGHT. AFTER A BRIEF ANALYSIS; WE ABANDONED THE APCH AS UNREASONABLE; UNSAFE AND HAVING BEEN UNNECESSARILY BEEN PUT IN A BAD SOT BY ATC EVEN TO HAVE JUST GONE IN TO LAND. IN THE ENSUING VECTORS BACK AROUND TO LAND; ATC MISSED HANDING US OFF TO TWR; SO WE JUST SWITCHED OVER WHEN BOUT ON A 3 MI FINAL TO THE ASSIGNED RWY.