Change of pre-paid meter to quarterly meter

Other

The tenants have vacated from the property I own. The property has a pre-paid gas meter (which was already installed when I purchased the property in May 2006.) The electric bill was paid quarterly. Gas and electricity are both supplied by Powergen -e-on.
I have contacted e-on this afternoon to request to remove the pre-payment meter.
This would seem to be a difficult process.
They have stated they would only remove it if the tenant requests this after being a customer for 12 months at the property
OR
I set up a direct debit and become liable for any tenant's future useage of gas (ie I become a guarantor) with an expectation I claim the money back direct from the tenant or include an amount in the rent.
Powergen have stated it is £50 to remove the meter. They have also said there was a change in their policy about 12 months ago regarding removal of pre-payment meters.
One of the reasons they do this is for their protection in payments and risk of new tenants not paying bills. I feel this is unfair and I am not sure if this is common practice for utility companies.
I feel it is an inconvenience to any of my potential tenants. Powergen offer a 3% discount when setting up a direct debit to pay the bills. Any potential tenant does not have this option.
Do you know if this is common practice that landlords have installed pre-payment meters and I should keep mine at the property?
If I wanted to challenge Powegen's policy on this can you advise me how to do so please.

I would appreciate your advice on this please.

Kind regards

Emma