Right. If you are looking "blocky" anyway and your main problem is roots, I would suggest you live with a "block" colour and use a permenant Blonde colour. You are a good candidate for one, as a very fair person naturally (anyone who is still "dark blonde" naturally in their 20's/30's/40's or beyond is pretty fair in my opinion)
My suggestions for colour would be as follows:
Clairol Nice N Easy Natural Extra Light Beige Blonde (can't go wrong, not too golden, not too ash on your ends, just a bog standard "blonde") or 103 Natural Light Beige Blonde.
Why "beige" - don't be put off, its in case there is quite a difference between your natural roots and the white bleachyness of your highlights. There is enough "colour" in these to be able to both lighten your roots adequately and deposit colour, and deposit colour to match in the bleached ends. You shouldn't end up with "root glow" or a muddy colour to your ends.
I suggest you have on standby (or be prepared to buy) some Lee Stafford "bleach blondes" shampoo or his "hot shots" to "tone" your colour if it comes out a little brighter than you like. Don't worry though, this will fade.
Its also not so light a colour that you can't then go to the hairdresser in the Autumn when you are more flush and have some highlights and lowlghts threaded through to do a more multi tonal look if you wish. But you might like it so much you wont bother!
Please do a strand test. Snip a small section of hair from the underside (say somewhere above your ear on either side), secure it with a sticky label or sticky tape, paint some of the colour (mix a teaspoon of each bottle as Clairol is equal parts), place the hair on top of some aluminium kitchen foil, fold it up in a little packet, wait 30-45 minutes, rinse, dry, and observe.
If you do this strand test you will not be shitting yourself when you are sitting there with the colour on your head! You will have a good idea of what it will look like, and you can sit there and stare at it if you like. When you choose the piece of hair for your strand test, please try and choose one of the baby blonde streaks that has been bleached, and try and snip it close enough to the root that you also get a section of "natural" hair - secure the hair at the end instead of the root with the sticky label so you can observe any difference in colour between the bleached blonde bit and the natural root bit.
Good luck, let us know how you go, and if you think this colour is wrong, say so and say why and I'll see if I can suggest an alternative.