How did you connect your surround sound source material to your old TV? If it was via scart, or TV aerial, it was not 5.1 surround sound. Neither of these are capable of transmitting digital 5.1 surround sound, only Nicam stereo (or Dolby Pro-Logic) at best. Unless you had either an optical or coaxial audio lead, or HDMI, connecting your source to the TV, and unless the source was DVD or Sky+/Sky HD, you definitely weren't sending a 5.1 surround sound to your TV. Analogue TV and digital Freeview, are not transmitted in 5.1 surround sound either. Being able to hear a noise coming out of your rear speakers when watching Eastenders does not mean you are hearing 5.1 surround sound.
DVDs, Sky+ and Sky Hd give you DD5.1 surround sound (and DTS on some DVDs). Your old TV was unlikely to produce 5.1 surround sound, it probably exported Dolby Pro-Logic surround sound to its own left and right speakers, a built-in sub-woofer, a centre speaker and two rear speakers. This was not 5.1 sound, it was a stereo signal expanded to give you analogue surround sound, but not DD5.1. Both rear speakers were outputting exactly the same sound at the same time. In true 5.1 surround sound, the rear left speaker and the rear right speaker run totally independent of each other.
In terms of buying an amp and connecting your LG TV to it - an AV amp or receiver will almost certainly take a stereo signal from your TV, as long as your TV has the red and white audio out connectors, and play back the signal it gets in Dolby Pro-Logic. Which will be no worse than what your old TV was capable of, and will probably be a lot better. With your old TV, the front left and right speakers were almost certainly connected to your TV, so only separated by the width of the TV screen. With a real surround sound setup, you can position the front speakers as wide apart as you like, so that sound from the front left speaker actually comes from the front left part of your living room, rather than just the left hand side of your television.
It will also allow you to connect your DVD player and your Sky+/Sky HD box to it using coaxial and optical connectors, to produce real DD5.1 sound.