With regard to your state pension, the plans to move to a flat rate (higher than the current basic state pension) from 2016 include a move from 30 to 35 qualifying years of service required. If you end up short of the 35 years, you should be able to purchase extra years of service via additional NI contributions.
This would be well worth checking out and potentially considering once the government confirms the cost. Meanwhile, if you wish to make additional provision towards retirement both Isas and a workplace pension could be suitable vehicles.
When it comes to tax advantages, pensions usually have a slight upper hand but there's often not much in it. Pension tax benefits are mostly upfront, that is contributions benefit from tax relief on the way in so a £100 contribution ends up costing you £80 after 20% basic rate tax relief.
When you eventually take your pension, up to a quarter of the fund can be taken as a tax-free cash lump sum while the remainder is used to provide a taxable income. By contrast, Isas offer no tax benefit on the way in, but all subsequent Isa income escapes tax. And while the monies are invested, both pensions and Isas benefit from the same tax treatment – that is no tax on income or gains.
Isas are far more flexible: you can take an income and/or withdraw money whenever you wish. Pensions are more strict: you can't touch the money until at least age 55 and, after any tax-free cash is taken, the balance must be used to provide an income, either via buying an annuity or leaving the money invested and drawing an income.
Another factor that may influence your decision is whether your employer pays money into the pension as an added perk. If they do, this may make the pension route far more attractive. If not, then you'll have to judge whether flexibility or tax efficiency is more important to you. And whichever you choose, pay close attention to charges and the underlying investments to ensure you get a good deal and don't take excessive risk.