Unknown British show from possibly the late 90’s

So all I remember about g is snow was effectively this this guy and two Kids running through this green tunnel that’s slowly turning red behind them the end up coming out of a grandfather clock that closes behind them then vanishes the last shot is of an old man and woman looking at each other then the credits role

The only other detail I remember is that said green tunnel lead to Buckingham palace

If anyone can tell me what the heck this was please di