Might beworthwhile proving how much electricity you are using. I see you mention £'s and not $'s.
Im in UK too, Electricity and Gas prices have sky rocketed recently, my bills have doubled but my consumption only went up 10-20%, mostly due to winter conditions.
Try eBay, but B&Q sell a plug in electricity monitor that tells you exactly how much electricity and cost is coming out of one socket...you might be able to prove its not all down to the fish tanks.
What size is your tank, what power of units are you using, i.e. what is the total wattage of
your filtration
any air pumps
heater
lighting
anything else plugged in
Look up the electricity website, you can find out pence per kW/h.
If you need a hand working out, post here and I'll give you a hand to work out your actual running cost.....
And your Dad might lay off if you can help the Hobby pay for itself - do you have an LFS that will exchange fish for goods. If so, you could easily breed Guppys, Danios, some Cichlids in exchange for the goods you need?