An abandoned vehicle is one which appears to have been left without an owner. It may not have been moved for a long time and it may be in poor condition, vandalised or in a dangerous position.
Tax discs are no longer needed. The vehicle tax could be up to date even if it doesn't display a tax disc or if it is displaying an out of date tax disc. Check whether the vehicle is taxed and has an up to date MOT (where applicable) using Gov.uk: check if a vehicle is taxed.
With the introduction of the End of Life Vehicle Directive, there is now a vehicle take back scheme. The Scrap my Car website can give you more information.