Honestly, I can't think of a viable upgrade for 12quid.
As riccardan has already said, 2nd gen is a bit too old to recommend at this point - still viable, but I would suggest looking a bit newer.
What are you using the rig for? Gaming/general use? No editing, rendering etc?
If so, look for an i5 opposed to an i7, as they'll be a bit cheaper.
Have a look locally & see if you can source a board + a Haswell i5 (4460, 4590, 4690k etc).
The majority of the Haswell or even Skylake i5s run you between 150-210 for the CPU new, and boards will vary depending on your needs.
Sticking with Haswell should allow you to reuse your DDR3, which saves a little bit over buying DDR4 (although you'll have to buy DDR4 eventually).
These are the cheapest upgrades I'd recommend - they're new prices of course, so maybe you could find a deal on barely/used.
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Core i5-4460 3.2GHz Quad-Core Processor (£150.00 @ Novatech)
Motherboard: MSI H97 PC MATE ATX LGA1150 Motherboard (£65.81 @ Amazon UK)
Total: £215.81
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-04-13 15:13 BST+0100
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Core i5-6500 3.2GHz Quad-Core Processor (£169.00 @ Amazon UK)
Motherboard: MSI H170A PC Mate ATX LGA1151 Motherboard (£80.99 @ Ebuyer)
Memory: Crucial 8GB (2 x 4GB) DDR4-2133 Memory (£27.99 @ Ebuyer)
Total: £277.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-04-13 15:15 BST+0100