President Reagan will stop in Helsinki, Finland, en route to the May 29-June 2 Moscow summit meeting and has arranged an overnight stay in London on his way back home. Reagan has accepted an invitation from Finnish President Mauno Koivisto to visit Finland May 26-29 before heading for Moscow. He will meet with Koivisto and also deliver a major address on prospects for a successful fourth summit meeting with Soviet leader Mikhail Gorbachev.