The CBSO have a one week only sale.
You can get 25% off tickets for most* concerts booked from now to Sunday 22 January, including the one with Jonas Kaufmann on 7 March (a solace for anyone who missed out on the Wigmore Hall date?) The concert includes a generous number of Strauss and Mahler songs (probably as much singing as you'd get in a headlining show). Tickets start at £8 with the offer - and with train fares from London starting at £10 return, it's possible to do the whole trip for less than a single Wigmore Hall seat!
(*The offer excludes: 3 March – Tristan and Isolde; 12 April – The Dream of Gerontius; 30 May – Britten’s War Requiem; 12 & 13 June: Symphony Hall 21st Anniversary Concerts.)