If a solicitor requires the copies of your documents for a conveyance then another solicitors certified copies would normally be acceptable. The other suggestions ie doctor, police officer, JP, would not , as their credentials can't be verified. (They are acceptable for photograph authentication when applying for a passport). The solicitor needs to see these documents under the Anti Money Laundering Regulations.
It should cost very little to get your certified copies. I work for a firm of solicitors we charge �1 per certified copy