Normally this is fine, and it may be preferable to adding to a long tiring day. Here's the however. The MAN-LHR shuttles aren't renown for timekeeping for various reasons and so the risk of a misconnect is there. BA will rebook you, either on the VS service or the late BA service BA239. So a big part of this is "what are the consequences of missing this flight and arriving at 8pm rather than 5pm? And a worse case scenario of being a day late? If it's in the nuisance-but-not-earth-shattering category then I'd take the risk, I had a very pleasant and on-time experience in MAN last week. If you're getting married the next day then that's rather different. If you do decide to rebook, unless some other factor applies you will have to get BA to remove the sector and requote the fare. And that may be expensive.