You can price the work as you see fit, if you want to put a provision in when you price up work on the expectation that you are going vat registered at a later date then there is no problem with that, it saves you then going back to the customer and telling them the price has gone up by 20% due to VAT. In the meantime you are making 20% more on those cleans.
However, you cannot show on an invoice that there is a 20% surcharge for VAT if you are not vat registered and your VAT number needs to be on all invoices, in most peoples cases this will only apply when working with commercial customers who are themselves VAT registered and need to claim back their vat expenses.
Also once you go VAT registered you can claim back any VAT costs that you have incurred in the past 3 years for any capital items that you have purchased, ie vans, systems, etc