Another possibly better option is to get an AC inverter which you can run from a Car battery -- 12 V DC input ---220 V AC output (as you said 70 GBP I assume UK so electricity supply there (when it's actually working !!) like rest of W/N/E Europe is 220 V not 120 V). This way you can charge the laptop while driving , use the laptop "out in the field" ON MAINS !! and being a 220 V AC you could plug in two other laptops at the same time - I wouldn't actually USE 3 laptops at the same time though on the inverter as you mighty find the Car battery would run down fairly quickly.
(Charging 2 and using one would be fine though).
Here are a load of 150 W one's -- from around 12 GBP to 40 GBP well under your budget --- and to use just plug the inverter into the car cigar lighter socket and your laptop into the standard AC outlet.
Enc pic as well as a link since some people might not know what an "Inverter" is given the lack of any sort of Engineering knowledge these days in Schools etc.
This is the one I use the BESTEK (pic shown) -- as it's 300 W output and even has 2 USB slots for charging USB devices like mobile phones which charge via USB slot (not USB mini==>micro adapters are also available). I have a decent battery in my car so the 300W is fine for me.
Costs 25 GBP and I use a 3 pin ==> European 2 pin adapter as the UK has its 3 pin rather non standard system.
For more choiceslook at the link Amazon.co.uk: 12v car inverter
Much more flexible since 150 W will EASILY CHARGE 3 laptops while running one as well.
( If you don't have a car then just get a separate 12 V car battery which you could charge at home and connect it to the inverter with a pair of Crocodile clips -- but I wouldn't imagine taking a Car battery on a Bus and charging a laptop while on the vehicle !! -- in any case any sort of Power reservoir that needs what you want won't be easily portable so I assume you have a car or a colleague who has one).
One of my part time gigs is shooting Nature photography in remote places -- power isn't usually available in these places so the Inverter is a perfect solution for me too.