It's only one poll and such a jump hasn't been seen with other polling companies but it is a significant result nonetheless. A party needs 20%+ of the vote to be reasonably confident of getting multiple MPs elected under the Victorian FPTP system.
In Wales, where UKIP is tipped to make the most significant gains, the party is currently sitting on 17% for the constituency vote and 16% for the list vote with YouGov and those figures are fairly consistent across polling companies. To put that into perspective, UKIP got less than 5% in the 2011 Welsh Assembly elections. Interestingly, the Tories are only on 19% for the constituency vote and 20% for the list vote whilst Plaid are on 21% for the constituency vote and 20% for the list vote. The vagaries of proportional representation mean that a 1-2% difference in vote can mean a 100% difference in seats (it's still an infinitely better system than FPTP though) but with these numbers it's possible that UKIP could end up the second or largest party in the Senedd next week.