The Finance Act 2013 introduced the R&D expenditure credit ("RDEC") from 1 April 2013.
From 1 April 2016 any company with qualifying R&D expenditure that cannot claim under the SME scheme can only make an RDEC claim.
The changes in the rules not only affects large companies, but alos SME's that cannot claim under the SME regime. There could be several reasons why an SME might not qualify for the SME scheme - for example if the R&D expenditure were subsidised, or the aid limit reached or if they were doing the research as a subcontractor.