If you are willing to use some avios (and find availability; also a problem on cheap fares) then any of the band 1 routes look good value. Off peak 15,500 avios + £50, or about 8,000+£130 avios&money (or about 3,000+£180 if you have an active IB+ account or some Amex points to activate it).
Some longer options such as MIL, NCE, MRS, BIO, GOA, SZG, MUC, PRG. Prague and Marseille go from T3 which has better lounges than T5.