I wouldn't recommend registering for VAT until you need to, generally the only types of business who do this are exporters who pay vat on their purchases but don't get vat back from their exported sales.
For a window cleaning business flat rate is the way to go and you can stay with it up to £185,000 turnover. The primary reasons for being flat rate are your costs are pretty low in comparison to your income and you can still claim the VAT back on purchases over £2000.
You charge your customers vat at 17.5%, you only pay the vatman back 10%, only downside is you can't claim vat back on smaller purchases, fuel, equipment etc, but the 7.5% difference should still work very much in your favour.
Finally the best reason is that it is very, very simple to do.
Do a calculation on your last 6 months figures and see if you would be better off flat rate or standard rate vat.