Ianto said 'I love you' in lingering looks and smiles, and the tender way he brushed back Jack's hair when it stuck to his sweaty forehead after they had sex. Ianto would mumble it into Jack's hair as he held him close, waiting for him to start breathing again. He mouthed the words against Jack's lips or the palm of his hand, tracing heart shapes on his chest with an idle finger.
He gasped it out with the last air in his lungs while he died in Jack's arms.
Jack said 'I love you' in the gentle kisses he pressed to Ianto's forehead, the pout on his lips and the sparkle in his eyes when he all but ordered him to stay with him for the night. Jack whispered it almost silently when Ianto slept soundly in his arms, and murmured it a little louder when he had nightmares, hoping to make them go away. He kissed the words into Ianto's neck and kissed them down his chest, a smile never leaving his face.
He never regretted anything more than not saying so Ianto could hear before he died.