Investors on Britain, Ireland and Denmark will benefit soon from liberalized exchange control rules approved by the British Treasury and the European Community.

The British Treasury announced today that it is dropping the 25 per cent surrender requirement on investment premium dollars, although the investment currency poll will remain in effect.

Under the changes, which become effective Jan. 1, British investors wishing to purchase foreign securities must first obtain foreign exchange from the pool of "premium dollars that are currently trading at about 90 per cent above the official pound-dollar exchange rate. When the securities are sold all of the proceeds may be converted to pounds through the premium dollar market. At present only 75 per cent of the proceeds may be converted at that market and 25 per cent must be converted at the prevailing commercial rate.

The new ruling will have the effect of making more premium dollars available and it will allow British investors to obtain a higher overall return on salws of foreign securities.

A dealer in premium funds said he expects the dollar premium to rise soon. Since the dollar premium is calculated on the basis of sterling's old parity with the dollar of 2.60571 those using the premium have been able to obtain a higher exchange rate than the present commercial rate. Thus while the premium is at about 90 per cent above the spot rate the effective surcharge to the investor is around 37.63 per cent taking into account the 2.60571 conversion rate.

The commission also directed Denmark to allow its residents to acquires securities issued by the EC and the European Investment Bank, such as Britain is planning.