Disclaimer: I am not a tax expert. Another disclaimer: because Pepone mentioned the company store and it amuses me, I'll refer to the pseudo-pounds you have in your value account as "company scrip". Yes, it is illegal in the UK to pay employees in company scrip, but since you have the option here to cash it out, that's not what they're really doing.
If the things you get are an actual benefit with value, then even though you've bought them with company scrip, they're still a benefit of employment because having the company scrip available to you in the first place, that is to say having the potential to get these things, is a consequence/benefit of employment.
This should be obvious in the case of any cash that you buy with your company scrip and that's added to your paycheck. At least, I hope it's obvious that this is employment income: it's certainly obvious to HMRC that it is!
The same goes if you use the company scrip to buy insurance. If that insurance would be considered an employment benefit for any purpose (for example tax, or compensation for wrongful dismissal), then the fact that a company scrip system was used to let you decide the nature of the benefit is irrelevant, it's an employment benefit. The fact that you have the option to take cash instead, though, means that anything you get this way will never be what HMRC calls a "trivial" benefit in kind, so the system can at least hypothetically have a small effect on tax compared with if the stuff just landed on your desk one morning unsolicited :-)
These systems often let you use the scrip for certain things that in fact aren't taxable or at least not at your marginal rate: pension contributions, childcare vouchers, buying a bicycle through work, whatever other schemes are in effect in the UK at the moment. So they still benefits, just not necessarily taxable ones, and that's one reason for presenting the value account balance to you as a gross figure.
Where the thing you buy is taxable, then it will be taxed at (an estimate of) its actual value, which is not necessarily equal to what they charged you for it in company scrip. It might be worth more, it might be worth less. It's going to be someone's responsibility (ultimately yours, I suspect, but maybe your employer's) to provide HMRC with a fair value to calculate your tax on. If this is all done through PAYE then you can pretty much ignore it if you choose, but it will affect your net paycheck. If you take the full 25% in benefits, and all those benefits are taxable at your maginal rate, which let's say is 40%, then it'd cost you 10% of your gross salary in additional deducted income tax, compared with what you'd pay if you had the same basic salary and no extra 25% benefits. The same goes for employee NICs.
So, if the mandatory insurance was not insurance, but instead a pig, and if you pay income tax at 40%, and suppose you really didn't want a pig even at 60% off retail price, then you'd want to decline the pig in order to avoid the income tax liability. Poor little piggy. Since you want to boost the insurance anyway, though, this isn't a problem for you.
Finally, if you use it to buy time off then I strongy suspect and fervently hope that this then is not taxable in any way, since really what you're doing is less work for less pay. A day off isn't a benefit in kind, although if they charge you less for it than 1/N of your salary, N being your normal number of working days in the year, then of course whatever you're being paid while on holiday is taxable like any other income.