Oops, no. The American tax year is the calendar year. The UK tax year runs April to April. You cannot file US forms on the UK tax year, and vice versa.
Now, the good news - you do NOT need a P60, nor any other document to serve as a W-2. They actually kind of take your word on it. There is also the little matter that in the US you report your gross income (before any payroll or other taxes are taken out), whereas in many countries (don't know about the UK) your "taxable income" is generally already adjusted for at least some of the social insurances.
So, under the Streamlined Compliance, you would file the current year (i.e. calendar year 2014) plus 3 prior years (2013, 2012 and 2011). OK, the other good news is that, as an overseas resident, you get an additional two months to file - so your deadline for a timely filing of 2014 is June 15th 2015, not April 15th.
Go here and play around with the software a bit to see how it works: TaxACT Free Online Tax Filing, File Free Online Tax Returns, Free Taxes Online for Everyone
Although you do have to register on the site to get access, if you give up in disgust, you can simply abandon the whole thing - or, you can print off what you've got (for free) and work with that (mail it in, or use it as a basis to fill the forms out by hand).
The one "trick" I'll advise you on is that you should NOT fill out a W-2 form when it asks for your salary. Scroll down the selection until you see a choice for Overseas or Foreign salary income. That should make the other options you need pop up automatically.
On your salary, you'll need to accumulate the calendar year amounts yourself (just the gross salary - none of the deductions are included in the US forms). Either do this from your copy of your payslips - or take your P60, back off your gross salary from January through April 6th 2015 and add back your gross salary paid in January through April 6th 2014. (If you haven't had a raise or other change in your salary amount, just take your monthly salary before deductions and multiply it times 12.)
The software runs from a questionnaire and you can access various definitions and help information when you run into something that stumps you. (Or you can come back here and ask.)
If you prefer, and your income is less than $60,000 for the year (you have to convert from GBP to US$ yourself - Yearly Average Currency Exchange Rates
) go to the IRS e-file site and pick a Free File provider: Free File: Do Your Federal Taxes for Free
(Just be aware that not all providers can deal with foreign taxpayers - check the restrictions posted for the provider.)