Given that the UK doesn't have a constitution, it clearly can't have amendments, hence can't have a 5th amendment.
IANAL, but I believe there is no legal defence concerning self-incrimination, but you do generally have the right to remain silent. Doing so, however, now allows the court to draw conculsions from your silence.
Some years ago, for example, the warning given to suspects when arrested changed to: "You do not have to say anything, but it may hurt your defence if you do not mention something you later rely upon in court."
The UK _does_ have a constitution -- but according to a barrister of my acquaintance it consists of thirty-one separate acts of parliament. (Think of it as a distributed legal kernel rather than a monolithic one. Oh, and the acts can be revised/amended by parliament without needing a constitutional convention or equivalent.)
There's no actual fifth amendment equivalent, but there _is_ a bill of rights (at least, since 1998). RiderOfGiraffes is exactly right about silence being a factor that can be drawn to the attention of a jury ...
On the subject of the bits of the Regulation of Investigatory Powers Act (2002) that make failure to hand over encryption keys an imprisonable offense, it's worth noting that an order to hand them over has first to be made in the process of a criminal investigation. If someone has encrypted data and refuses to hand over the keys despite facing a maximum 5-year prison term, then it's reasonable to presume that they consider the data to be so incriminating that they'd pull a _longer_ sentence if they decrypted it. I believe (but am not certain -- the powers have been used to rarely that there's little to go on) that a plausible explanation of why the keys are unavailable ("I generated a PGP keychain in 1996 out of curiousity, but I lost interest and stopped using it, and that was twelve PCs ago") would probably work in court (in the absence of evidence contradicting it).
The court can draw inferences from your silence, but generally there would need to be further external evidence (circumstantial, hearsay etc) for those inferences to mean anything.
Unfortunately in this case, the offence is complete when you don't say anything (don't give up the key(s)).
And just for completeness, the caution in the UK is:
"You do not have to say anything, but it may harm your defence if you do not mention, when questioned, something which you later rely on in court. Anything you do say may be given in evidence."
There was always a presumption of innocence which like a lot of classical rights has not been transferred to the digital realm. You do not have to say anything but your silence may be presumed as acknowledgement of your guilt.
IANAL, but I believe there is no legal defence concerning self-incrimination, but you do generally have the right to remain silent. Doing so, however, now allows the court to draw conculsions from your silence.
Some years ago, for example, the warning given to suspects when arrested changed to: "You do not have to say anything, but it may hurt your defence if you do not mention something you later rely upon in court."