You don't strictly need the share certificates in order to sell the shares, but you would need to complete a form of indemnity. If you are selling the holding through a bank or stockbroker, they will be able to tell you whether the completed Indemnity can be used to support the sale or whether a duplicate certificate will be needed.
Computershare nowadays seem to be the largest Company Registrar and also offer a share dealing service both via the internet and telephone. If they are the registrars for your holding, they should be able to advise you of the requirements and may be able to trade on your behalf, but you would need to compare their charges to those charged by other dealing vehicles. You would need to quote your Shareholder Reference Number which appears on your share certificates.
DocHH is partially correct regarding the taxation on any dividends received, as a 10% Tax Credit is deducted from your dividends, which is non refundable, but further tax would be due dependent upon your overall tax situation and marginal rate.
Do you get any worthwhile discount from the High Street shop as a shareholder perk, which may require a minimum shareholding?
If you Google 'Selling Shares' you should find many brokers listed who would undertake your transaction, at a price!