Carrying handbag/purse in major museums, Tower of London, what to expect regarding searches, etc.

Just about every major museum now requires a security checkpoint. There is no way to "avoid" it. They most likely will search all bags/purses, though my experience is that they just look in. Some museums will have metal detectors and/or xray scanners.

Purses are allowed in museums. But a larger bag or backpack type bag will often have to be checked in lockers or at a bag check.

If I carry a small bag, will I be less likely to have to go through a
checkpoint?

If there's a security checkpoint at an attraction anywhere in Europe, it has been our experience that no one is allowed to skip those queues. London security being especially on guard these days, you can count on having to go through the checkpoints.

Comments from visitors to the Tower earlier this year:
"The queue that "everyone" then has to go through, even if pre-purchased online or a tour, is for security check at entry."

"There's also a security check on entering the Tower - again that applies to everyone."

Westminster Abbey also has security checks:
"It is not possible to enter the Abbey for worship or as a visitor with large or bulky items of luggage. For clarity, this includes suitcases (including those on wheels) and large rucksacks. Left luggage facilities are operated at Charing Cross and Victoria stations, both of which are within walking distance of the Abbey. The Dean and Chapter also reserve the right to search smaller bags before granting access to worshippers and visitors."

http://www.westminster-abbey.org/press/news/2013/july/security-at-westminster-abbey

The Tower doesn't appear to provide bag-checking services, and the British Museum doesn't allow checking of larger bags/luggage.

What would be the point of having security checks if people could skip them?

What you want to avoid is carrying a bag large enough it has to be checked---there is a separate line cor that. This often includes all backpacks and larger shoulder bags. I remember the Villa Borghese being particularly strict.

The OP isn't going to Italy but Galleria Borghese is a special case where almost everything on you has to be checked, and they provide a service for it...which is why you're required to be there 30 minutes or so before your time slot.

Not all attractions offer checking services so it's always best to take smaller versus larger. St Paul's Cathedral, for instance, doesn't have any sort of cloakroom/bag-checking facility.

I recall many years ago having to check my bag (small to medium size) at a museum in Amsterdam, but don't think the size mattered. I think all bags were being checked into lockers. Don't remember which one, but this was before security was as tight as it is today. I know I am overthinking everything, but I read things like "always carry an umbrella, dress in layers, etc.", then I come across sites that indicate there is nowhere to leave anything which means you will be carrying everything around with you all day long. I tend to be a planner and be-prepared (as in others expect me to have mints, tissues, sewing kit, etc.) kind of person, but, then, maybe that's why I'm on this forum asking questions. I appreciate the patience and suggestions of more experienced travelers than myself.

The restrictions on bags often has more to do with security of the collection/museum than with security against acts of terror. People carrying larger bags are often careless with them, and swinging them around can knock into art objects or finishes.

Yes, most places search bags in large tourist attractions. They are fairly efficient about it, so don't worry overly about time. Though, the smaller the bag, the shorter the time to search.

Sometimes, they'll only search larger bags or backpacks. I definitely got waved through a couple times with my side bag, or just a quick peek in the top. I usually carry this one, if it gives you an idea about size: http://www.zappos.com/p/dooney-bourke-nylon-crossbody-black-with-tan-trim/product/8293769/color/211584?zlfid=191&ref=pd_detail_1_sims_p_ab

I have never been asked to check it in the UK.

My experience, last Sunday afternoon going to a concert at the Royal Albert Hall, was security outside before the ticket entrance. Our bags were all searched. In my case I had a very small handbag with shoulder strap and the guard had a good look at the contents. I had also had an email from the venue about a week before warning that there would be increased security and suggested arriving in plenty of time. The RAH has not recently had security at this level.