Popular MVNO giffgaff is welcoming OVIVO refugees (RIP OVIVO) with a bonus fiver if you sign up before the end of April. In addition, all SIMs ordered before the end of Sunday March 23 will be sent out first class rather than the usual second. Lovely.
Bear in mind that OVIVO used the Vodafone network, while giffgaff piggybacks O2.
(Thanks to vindico at HUKD)