The cheapest way is to rent your roof to a company who install the panels, take the Feed In Tariff payments and give you free electricity during production hours. HOWEVER, with the FIT scheme currently set at 20 years, you normally have to sign a lease and effectively you have what’s known as a “Sitting Tenant”, with all of the legal rights that come with that.
I went for the buy-outright option. For an initial outlay (at the time in the region of £6,000 for a 3.75kW system) in the south east I have an annual FIT income of about £800. As the tariffs have dropped so have the price of panels and anyone I know with panels expects to break even after about 7-8 years, which may have a bearing on your decision to install just before you move. I’m not sure how much the panels would add to the property value, but the tariff can be transferred if you sell so the new occupiers would benefit from a potentially higher rate.
If you do decide to install (and depending on your hot water set up), it may be worth looking at a device to divert excess generated electricity to your immersion heater during the day. I installed one along with another layer of insulation and my boiler hasn’t really been on since May. I went for the Intelligent Immersion model, but there are a few out there.
Hope that all helps!