The Pacer multiple-units (classes 142/3/4) are Britain's most controversial trains these days (unless you count railway lines yet to be built like HS2) with urging at the highest levels to withdraw these ageing units from service yet they continue to serve in places like South Wales and the North of England.
The Pacer concept was a development from Britain Rail's railbus ideas of the 1970s. This paired the Leyland National bus body with a rail chassis to produce a cheap unit for rural lines which would enable these lines which were under financial pressure to remain open. The problem with the Pacer however was the chassis, this was a development of a chassis designed for high speed freight in the 1960s. Hence the Pacer units have always suffered from poor rail riding with a rough ride and excessive squealing of the wheels especially over tightly curved track and points.
The Class 142 and 143 Pacers are (probably) on borrowed time now as new accessibility regulations will make operation of them illegal past 2020 unless they receive extensive (and expensive) modifications. It is unlikely there is the will or money to do that, especially as the continued use of the Pacer has become a political hot potato. However they will be over 30 years by then and so due for retirement anyway. Some of the class 144s may survive as work is being done on an improvement package however as electrification of Britain's railway network continues other newer diesel multiple-units are being made available to be cascaded elsewhere so there may no longer be a need to keep the Pacers going beyond the end of the decade.